! Fix several PM label bugs

This should, hopefully, be the last of the PM label bugs...

! Trying to remove the inbox label didn't work (fixes #1351)
! Trying to view all PMs with a particular label caused a database error when not in conversation view
! Applying a label in conversation view only applied it to the first PM in the conversation (fixes #1358)

Signed-off-by: Michael Eshom <oldiesmann@oldiesmann.us>