When deleting all messages in a conversation, the entry in the left pane
should be inserted into the alphabetical portion of the list. To keep it
in this collection, do not nullify active_at.
To ensure the list view is keeping itself correctly sorted, make sure
that resorting behavior is triggered any time a relevant attribute is
changed.
This fixes deleted conversations jumping to the top of the list, and
conversation order scrambling when getting a group or contact sync
message from our master device.
Fixes#734
// FREEBIE