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)

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.

Mautrix whatsapp

Regression since 67a445a74a

It's been incorrect since 23e4a473

Expose mautrix-telegram public endpoint through nginx

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

Add DNS records table

Add sample .well-know config for Caddy

Add missing tag to task

The setup-all should include the setup_well_known task as well

Fix link to using your own webserver

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

Fix minor typo in install instructions

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

Coturn reports it as an option that conflicts with `use-auth-secret`.
Some reasoning is here: fa523e8d09

`--log-driver=none` is used for all Docker containers now.

All these containers are started through systemd anyway and get logged in journald,
so there's no need for Docker to be logging the same thing using the default `json-file` driver.
Doing that was growing `/var/lib/docker/containers/..` infinitely until service/container restart.

As a result of this, things like `docker logs matrix-synapse` won't work anymore.
`journalctl -u matrix-synapse` is how one can see the logs.

If the playbook were to run with `--tags=setup-nginx-proxy`,
it wouldn't go into `setup_corporal.yml`, which meant it wouldn't
perform a bunch of `set_fact` calls which override important
nginx proxy configuration.

We run these variable overrides on each call now (tagged with `always`)
to avoid such problems in the future.