It looks like SELinux can be left running without any (so far) negative
effects on our Matrix services.

There's no need to use `:z` or `:Z` options when mounting volumes either.
This means that files we create are labeled with a default context
(which may not be ideal if we only want them used from containers),
but it's compatible and doesn't cause issues.

Relabelling files is probably something we wish to stay away from,
especially for things like the media store, which contains lots of
files and is possibly on a fuse-mounted (S3/goofys) filesystem.

The new image is built in a much better way (2-stage build)
and is 10x smaller.

In terms of Goofys version recency, it's about the same..
Both images (and others alike) seem to not use version tags,
but rather some `:latest` (master), with ewoutp/goofys being a bit
more recent than clodproto/goofys.

Not using version tags is good (in this case),
because the last Goofys release seems to be from about a year ago
and there had been a bunch of bugfixes afterwards.

This is described in Github issue #58.

Until now, we had the variable, but if you redefined it, you'd run
into multiple problems:

- we actually always mounted some "storage" directory to the Synapse
container. So if your media store is not there, you're out of luck

- homeserver.yaml always hardcoded the path to the media store,
as a directory called "media-store" inside the storage directory.

Relocating to outside the storage directory was out of the question.

Moreover, even if you had simply renamed the media store directory
(e.g. "media-store" -> "media_store"), it would have also caused trouble.

With this patch, we mount the media store's parent to the Synapse container.
This way, we don't care where the media store is (inside storage or
not). We also don't assume (anymore) that the final part of the path
is called "media-store" -- anything can be used.

The "storage" directory and variable (`matrix_synapse_storage_path`)
still remain for compatibility purposes. People who were previously
overriding `matrix_synapse_storage_path` can continue doing so
and their media store will be at the same place.

The playbook no longer explicitly creates the `matrix_synapse_storage_path` directory
though. It's not necessary. If the media store is specified to be within it, it will
get created when the media store directory is created by the playbook.

Previously, it was more necessary to have it
(because we had a dependency between matrix-synapse and matrix-nginx-proxy)..
But nowadays, it can be removed without negative side effects.

Restarting matrix-nginx-proxy is especially bad when the proxy is not installed at all.

Fixes a problem where importing would lead to this error:
    Cannot link to /matrix-postgres, as it does not belond to the default network.

Small bugfix related to #55

* Add default template file for homepage HTML
* Add default riot-web config options for homepage

mxisd supports several identity stores. Add support to configure two of them:

* synapseSql (storing identities directly in Synapse's database)
* LDAP

This removed the need to copy `mxisd.yaml.j2` to the inventory in case one wants
to use LDAP as identity store. Note that the previous solution (copying
`mxisd.yaml.j2` was poor because of two reasons:

* The copy remains outdated in case the original is updated in future versions
  of this repo.
* The role's configuration should be in one place (configured only through role
  variables) instead of in multiple.

Configuring more identity stores through role variables can be supported in the
future.

This is provoked by Github issue #46.

No client had made use of the well-known mechanism
so far, so the set up performed by this playbook was not tested
and turned out to be a little deficient.

Even though /.well-known/matrix/client is usually requested with a
simple request (no preflight), it's still considered cross-origin
and [CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS)
applies. Thus, the file always needs to be served with the appropriate
`Access-Control-Allow-Origin` header.

Github issue #46 attempts to fix it at the "reverse-proxying" layer,
which may work, but would need to be done for every server.
It's better if it's done "upstream", so that all reverse-proxy
configurations can benefit.

This makes it more consistent with other usage throughout the playbook.

As Github issue #42 says, 0.17.7 is not available.
We shouldn't have tried to use it.

This reverts commit 529b5b222d.

Trying to:

- stay closer to naming in Synapse (autojoin -> auto_join)

- not create new variable namespaces (`matrix_homeserver_`),
when existing ones (`matrix_synapse_`) are more suitable

- allow `null` (`~`) values for `matrix_riot_web_welcome_user_id`

- render things like `auto_join_rooms` in `homeserver.yaml` more prettily

- fix breakage in `config.json` where `matrix_riot_web_roomdir_servers`
was rendered as YAML and not as JSON

- simplify code (especially in riot-web's `config.json`), which used
`if` statements that could have been omitted

- avoid changing comments in `homeserver.yaml` which are not ours,
so that we can keep closer to the configuration file generated by upstream

This brings the list in line with the new default as of https://github.com/matrix-org/synapse/pull/4207

This also raises the default from 5 seconds to 15 seconds.

Regression since 3fd6fd647f

Pretty much all variables live in their own `matrix_<whatever>`
prefix now and are grouped closer together in the default
variables file (`roles/matrix-server/defaults/main.yml`).

It should be `/bin/mkdir` and `/bin/chown` on Ubuntu 18.04 for example.
Still, it doesn't seem like we need to create and chown these
directories at all, since the playbook takes care of creating them
and setting appropriate permission by itself.

Regression since 67a445a74a