Introduce connection string (database url) for roundcube.

Remove database choice from setup.
Remove the old *DB_* database env variables from the documentation.
The env vars are deprecated now. They will be removed after the upcoming
Mailu release.

core/admin/mailu/configuration.py

@@ -27,6 +27,7 @@ DEFAULT_CONFIG = {
     'DB_NAME': 'mailu',
     'SQLITE_DATABASE_FILE': 'data/main.db',
     'SQLALCHEMY_DATABASE_URI': 'sqlite:////data/main.db',
+    'SQLALCHEMY_DATABASE_URI_ROUNDCUBE': 'sqlite:////data/roundcube.db',
     'SQLALCHEMY_TRACK_MODIFICATIONS': False,
     # Statistics management
     'INSTANCE_ID_PATH': '/data/instance',