Create PurpleAuthorizationRequestNotification

This is the first discrete notifications objects after /r/3009. This also fixes
an issue in /r/3009 where PurpleNotification was still marked as a final type.

Testing Done:
I have a WIP branch for updating the Pidgin side that I used to verify functionality but it has some issues so isn't included here.

Also ran the unit tests under valgrind.

Reviewed at https://reviews.imfreedom.org/r/3035/