Chats should handle multi-protocols group chats

User story

My mum's contact as SMS (MMS) only, my brother's contact has MMS + Librem One, my sister's contact has Librem One only. I want to start a conversation with the 3 of them. It should go through Librem One for my brother and sister but my mum still should be able to follow the discussion through MMS as a fallback.

Design

The UIs workflow to handle group chats is described in the issue #235 (closed)