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* The copy remains outdated in case the original is updated in future versions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.

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

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

It used to allow user to logging with their own account to the bot
see: https://github.com/tulir/mautrix-telegram/wiki/Authentication#replacing-telegram-accounts-matrix-puppet-with-matrix-account
for more informations

We skipped v0.33.6 because of matrix-org/synapse#4014,
but v0.33.7rc2 fixed the problem.

Coturn reports it as an option that conflicts with `use-auth-secret`.
Some reasoning is here: fa523e8d09

`--log-driver=none` is used for all Docker containers now.

All these containers are started through systemd anyway and get logged in journald,
so there's no need for Docker to be logging the same thing using the default `json-file` driver.
Doing that was growing `/var/lib/docker/containers/..` infinitely until service/container restart.

As a result of this, things like `docker logs matrix-synapse` won't work anymore.
`journalctl -u matrix-synapse` is how one can see the logs.

Fix typo
Move mautrix variable in `defaults/main.yml` exclusively

I've found the previous 10K default value to be way too low
on a bunch of servers I'm running, so it's now up to
100K and made configurable.

Closes Github issue #5

Closes Github issue #3

Regression since a302a7d7489ec,
which made the Matrix Client API only available on
the http port (8008) and not over the federation port (8448).