Strings containing double-quotes would cause YAML syntax breakage.
Not anymore.

Closes #48 and #49.

For people using multiple password providers, it makes sense
to have the fastest one (which doesn't make network requests) be first.

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.

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

Regression since 67a445a74a

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

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

This disables federation on the 80 port, as it's
not necessary. We also disable the old Angular webclient.

For the federation port (8448), we disable the client APIs
as those are not necessary. Those can even cause trouble
if one doesn't know about them and thinks that guarding the client
APIs at the 80 port is enough.

Until now, we were starting from a fresh configuration, as generated
by Synapse and manipulating it with regex and line replacements,
until we made it work.

This is more fragile and less predictable, so we're moving to a static
configuration file generated from a Jinja template.

The upside is that configuration will be stable and predictable.

The downside of this new approach is that any manual configuration changes
after the playbook is done, will be thrown away on future playbook
invocations.

There are 2 ways to work around the need for manual configuration
changes though:
- making them part of this playbook and its default template
configuration files (which benefits everyone)
- going your own way for a given host and overriding the template files
that gets used (that is, the
`matrix_synapse_template_synapse_homeserver` or
`matrix_synapse_template_synapse_log` variables)