Fixes #191 (Github Issue).

Related to #189 (Github Issue).

People had proxying problems if:
- they used the whole playbook (including the `matrix-nginx-proxy` role)
- and they were disabling the proxy (`matrix_nginx_proxy_enabled: false`)
- and they were proxying with their own nginx server

For them,
`matrix_nginx_proxy_proxy_matrix_additional_server_configuration_blocks`
would not be modified to inject the necessary proxying configuration.

From what I see, this was never implemented to begin with.

Fixes #189 (Github Issue).

Fixes #171 (Github Issue).

Continuation of 54a281a425.
Related to #188 (Github Pull Request).

Validate_config was copied from the telegram bridge code, but doesn't
apply to the facebook bridge.

While using certbot means we'll have both files retrieved,
it's actually the fullchain.pem file that we use in nginx configuration.

Using that one for the check makes more sense.

Fixes #155 (Github Issue)

Reasoning is the same as for matrix-org/synapse#5023.

For us, the journal used to contain `docker` for all services, which
is not very helpful when looking at them all together (`journalctl -f`).

The goal is to move each bridge into its own separate role.
This commit starts off the work on this with 2 bridges:
- mautrix-telegram
- mautrix-whatsapp

Each bridge's role (including these 2) is meant to:

- depend only on the matrix-base role

- integrate nicely with the matrix-synapse role (if available)

- integrate nicely with the matrix-nginx-proxy role (if available and if
required). mautrix-telegram bridge benefits from integrating with
it.

- not break if matrix-synapse or matrix-nginx-proxy are not used at all

This has been provoked by #174 (Github Issue).

Use Jinja2 lstrip_blocks option in templates to ensure consistent
indentation in generated files.

This reverts commit 172d59ba05.

This prevents Ansible from sometimes failing to decrypt vault variables

As discussed in #151 (Github Pull Request), it's
a good idea to not selectively apply casting, but to do it in all
cases involving arithmetic operations.