This is a pretty pure code move, where we moved stuff from
message_store to pm_conversations:
insert_recent_private_message() -> recent.insert()
recent_private_messages -> recent.get()
The object message_store.recent_private_messages was not
encapsulated in a function before this change. Now it is
hidden in the scope of pm_conversations.recent.
Both of the modules touched here maintain 100% line coverage.
In pm_conversations.js, added function to make a user a PM partner and
another function to check if a user is a PM partner. A PM partner is
someone with whom the user has been in a PM with.
Steve Howell
Akhil