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* The copy remains outdated in case the original is updated in future versions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

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 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.

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

We've been using Goofys for a long time instead.