Make appservice-discord support both SQLite and Postgres

People can toggle between them now. The playbook also defaults
to using SQLite if an external Postgres server is used.

Ideally, we'd be able to create databases/users in external Postgres
servers as well, but our initialization logic (and `docker run` command,
etc.) hardcode too many things right now.

roles/matrix-bridge-appservice-discord/templates/config.yaml.j2

@@ -58,8 +58,11 @@ database:
   # If you are migrating, see https://github.com/Half-Shot/matrix-appservice-discord/blob/master/docs/howto.md#migrate-to-postgres-from-sqlite
   # WARNING: You will almost certainly be fine with sqlite unless your bridge
   # is in heavy demand and you suffer from IO slowness.
-  #filename: "/data/discord.db"
-  connString: "postgresql://matrix_appservice_discord:{{ matrix_additional_databases | selectattr('name', 'equalto', 'matrix_appservice_discord') | map(attribute='pass') | first }}@{{ matrix_postgres_connection_hostname }}/matrix_appservice_discord"
+  {% if matrix_appservice_discord_database_engine == 'sqlite' %}
+  filename: {{ matrix_appservice_discord_database_filename|to_json }}
+  {% else %}
+  connString: {{ matrix_appservice_discord_database_connString|to_json }}
+  {% endif %}
 room:
   # Set the default visibility of alias rooms, defaults to "public".
   # One of: "public", "private"