Make bridge permissions more easily configurable

Not doing {% if matrix_admin %} checks in the YAML also fixes some issues
with indentation being incorrect sometimes.

This should be backward compatible, except for mautrix-signal's case
where `matrix_mautrix_signal_bridge_permissions` previously existed
as a string, not a dictionary. `tasks/validate_config.yml` will catch
the problem an even provide a quick fix.

roles/matrix-bridge-mautrix-googlechat/defaults/main.yml

@@ -48,6 +48,12 @@ matrix_mautrix_googlechat_homeserver_token: ''
 # If false, created portal rooms will never be federated.
 matrix_mautrix_googlechat_federate_rooms: true
 
+matrix_mautrix_googlechat_bridge_permissions: |
+  {{
+    {matrix_mautrix_googlechat_homeserver_domain: 'user'}
+    | combine({matrix_admin: 'admin'} if matrix_admin else {})
+  }}
+
 # Database-related configuration fields.
 #
 # To use SQLite, stick to these defaults.