Move mautrix telegram and whatsapp into separate roles

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).

roles/matrix-synapse/vars/main.yml

@@ -4,3 +4,6 @@ matrix_synapse_id_servers_public: ['vector.im', 'matrix.org']
 
 matrix_synapse_client_api_url_endpoint_public: "https://{{ matrix_server_fqn_matrix }}/_matrix/client/versions"
 matrix_synapse_federation_api_url_endpoint_public: "https://{{ matrix_server_fqn_matrix }}:8448/_matrix/federation/v1/version"
+
+# Tells whether this role had executed or not. Toggled to `true` during runtime.
+matrix_synapse_role_executed: false