Make message handling more generic (adding in system messages)

- This sets us up for storing JOIN and PART messages (for groupchats)
  in the database.
  - These are stored as 'system messages', which are essentially
    somewhat hacky objects with various reference fields that
    are filled in depending on the type.
- A set of generic functions are used to transform some message-like
  database object into an IRC message to be sent down the wire.