[#96] Resolve "side chats"

Closes #96 (closed)

It has been a marathon 2-day sprint to get this done in the middle of an insurrection. I am too tired to write a proper MR message right now. I promise to circle back and write one later!

I QA'ed this with @fdbk and @mari and it meets all the below acceptance criteria:

GIVEN alice is an admin on channel [foo] in hotline mode

• WHEN bob sends the message we need a medic now! to the channel
  • THEN this message will be forwarded to all admins prefixed by a header that says [HOTLINE #1312] (in the language of each admin)
• WHEN alice sends a message to the channel that says REPLY #1312 where are you?
  • THEN both bob and all admins of the channel will be sent the message where are you?
  • AND the message seen by bob will be prefixed with the header [PRIVATE REPLY FROM ADMINS] (in bob's language)
  • AND the message seen by all other admins will be prefixed with the header [REPLY TO HOTLINE #1312] (in the admin's language)

GIVEN the passage of a 24 hour interval

• THEN a job will run that deletes all records of hotline message / sender tuples older than one month to preserve user privacy