If both users haven't posted in the current topic, then
as a second order sort, check which user has posted first
in the stream as a whole.
Fixes part of #5956; we still need to sort by sending in the
organization.
It appears that a regression introduced in
3f60074c33 caused undefined to be passed
as the subject to the recent_senders library much more often; this
fixing that, and makes the library handle such cases reasonably
without an exception regardless.
This was causing a huge number of "Tried to call a Dict method with an
undefined key." exceptions.
In recent_senders module, added a data structure to hold timestamps of
users' latest message in a topic. Also added a function to compare 2
users based on above timestamp. Added a function to process messages for
the data structure and a call in add_message_metadata. Also added node
tests for insertion of data into recent_senders.senders.
Cory Lynch
Tim Abbott
Akhil