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-twitter/defaults/main.yml

@@ -25,6 +25,12 @@ matrix_mautrix_twitter_appservice_address: 'http://matrix-mautrix-twitter:29327'
 
 matrix_mautrix_twitter_command_prefix: "!tw"
 
+matrix_mautrix_twitter_bridge_permissions: |
+  {{
+    {matrix_mautrix_twitter_homeserver_domain: 'user'}
+    | combine({matrix_admin: 'admin'} if matrix_admin else {})
+  }}
+
 # A list of extra arguments to pass to the container
 matrix_mautrix_twitter_container_extra_arguments: []