Fixed error message to direct users to the right debug command

Attempt to fix #192 (Github Issue), potential regression since
70487061f4.

Serializing as JSON/YAML explicitly is much better than relying on
magic (well, Python serialization being valid YAML..).
It seems like Python may prefix strings with `u` sometimes (Python 3?),
which causes Python serialization to not be compatible with YAML.

This doesn't replace all usage of `-v`, but it's a start.

People sometimes troubleshoot by deleting files (especially bridge
config files). Restarting Synapse with a missing registration.yaml file
for a given bridge, causes the `-v
/something/registration.yaml:/something/registration.yaml:ro` option
to force-create `/something/registration.yaml` as a directory.

When a path that's provided to the `-v` option is missing, Docker
auto-creates that path as a directory.
This causes more breakage and confusion later on.

We'd rather fail, instead of magically creating directories.
Using `--mount`, instead of `-v` is the solution to this.

From Docker's documentation:

> When you use --mount with type=bind, the host-path must refer to an existing path on the host.
> The path will not be created for you and the service will fail with an error if the path does not exist.

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).

Facebook API token not required

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

Fix ansible deprecations

Refers to #89 (Github Issue)

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)

Move bridges into separate roles

Add a variable to control the stop tasks

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`).

Fix case of the mxisd ldap.connection.baseDNs option in matrix_mxisd_configuration_extension_yaml comment

Provide a sample rest_auth_endpoint close to actual setup

Fix matrix_synapse_ext_password_provider_ldap_start_tls (it's boolean)