Simplify AugmentedMessage and AugmentedSession

Fixes #510 (closed) and #511 (closed)

This should drastically improve performance in the messages and session views.

TODO:

• Stop fetching all attachments in AugmentedMessage, only fetch attachment count (ensure that the triggers are still in place)
• Remove reactions from AugmentedMessage
• [ ] Consider removing receipts from AugmentedMessage, only store compressed form Not beneficial, since the extra query is needed either way.
• Stop fetching quoted messages
• Clean up Session::source and Session::message properties
• Further clean-up Session