* Add fields `protocol` and `group_id`

- The table is `conversation` table in Galley

* Introduce the protocol field to various conversation types

* Drop MLS from `NewRemoteConversation`

- A remote backend can never get a notification about a new remote MLS
conversation as the conversation is created outside MLS and only the
conversation creator is initially present. Members are to be added via
the MLS protocol subsequently.