Set default mautrix-telegram image to 0.4.0

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.

New scripts

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

Add LDAP auth support

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

This reverts commit 529b5b222d.

We've had some people get confused into installing
Matrix Corporal and having pain with that.

With this documentation change, we try to make it clearer
that it's an advanced feature not to be touched unless
you know what you're doing.

On a similar note, we also make sure other things are properly
labeled as "(optional)" and/or "(advanced)".

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

Remove riot.im from list of trusted identity servers

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

Fix well known self check

Allow Synapse log levels to be configured (#23)

Add initial version of maintenance and troubleshooting doc

Link to the restoring media store guide

It isn't below anymore

Updated "Controlling Matrix federation" link

Missing file ending results in 404

Allow a few parts of Riot config.json to be configured (#24)

Disable TLS 1.0