Implement Network Sync for Group Chats

[zicklag]

Group chats will be implemented almost identical to Direct Messages.

The differences will be:

• We don't bother with encryption for group chats.
  • In the future we will have group encryption through Beehive most-likely.
• It becomes much less efficient to download the latest snapshot of the automerge document from every known group member, like we do for direct messages, so we may need to limit group sizes and make a more sophisticated solution, possibly involving an always-on service for groups, and always on indexer to keep up with what group updates you need to download from whom, or a way to fragement the automerge documents into smaller pieces to reduce their download size from the PDS.