FEATURE: clicking envelope takes you to inbox
Suggested messages works somewhat like suggested topics.
- New show up first (in either group inbox or inbox)
- Then unread (in either group inbox or inbox)
- Finally "related" which are messages with same participants as the current pm.
This allows the discourse-tagging plugin to correctly use two rows in
the header if it needs to display tags, or one row if there are no tags.
This works in tandem with the same logic for when there is a category
badge to display or not.
