Implement sender realname/avatarurl storage

- Store sender realname and avatarurl in database
- Guard protocol changes behind feature flag 'RichMessages'
- Avatar URLs are not actually stored yet, as we do not implement
  METADATA yet.

This is not yet used in Quassel itself, but third-party clients
including QuasselDroid use this to provide more context in messages
and allow for realname-based fallback avatar fetching to work in the
chat message backlog.

Incorporates suggestions from @mamarley.

Closes GH-351.