Add more logging during message sending

- Join rules
- Guest access

fixes #257

The avatars are loaded from cache

Convert list of receipts to map<userId, timestamp>

fixes #107

fixes #128

fixes #154

Make Cache constructor exception free

fixes #74

* Initial "join room" feature.
* React correctly to remotely joined rooms.
* Leaving rooms implemented both locally using the room menu
   in nheko, and reacting properly when leaving a room remotely 
   from another client.