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

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

Regression since 3fd6fd647f

Allow specifying which parts of the role run from playbooks

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`).

Update riot-web (0.17.0 -> 0.17.3)