Put variables that can contain arbitrary strings into a block scalar. This way,
no escaping of quotes or backticks is needed.

This fixes some bugs. For example, putting a backtick into the variable
`matrix_synapse_macaroon_secret_key` previously led to an error when starting
`matrix-synapse.service`, so the service couldn't start. The error message was:

```
[...]
[...] docker[19227]: yaml.scanner.ScannerError: while scanning a double-quoted scalar
[...] docker[19227]:   in /data/homeserver.yaml, line 451, column 29
[...] docker[19227]: found unknown escape character '`'
[...] docker[19227]:   in /data/homeserver.yaml, line 451, column 37
[...] docker[19227]: Traceback (most recent call last):
[...] docker[19227]:   File /start.py, line 66, in <module>
[...]
```

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

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

The setup-all should include the setup_well_known task as well

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

If a network like `matrix-whatever` already exists for some reason,
the `docker_network` module would not create our `matrix` network.
Working around it by avoiding `docker_network` and doing it manually.

Fixes Github issue #12

The old way is deprecated and would stop working
after Ansible 2.11.