chatmail
Simplify and speed up `calc_sort_timestamp()`
This is separated from closed #7180 . Everything related to fixing #7308 is removed. "Messages are end-to-end encrypted." is reordered in test_outgoing_encrypted_msg(), but as there are corner cases anyway (see #7308 ), this isn't important and shouldn't block code simplifications. The current code is hard to maintain and slow, i.e. it looks at every message in the chat, this doesn't scale.
Tested on some random chat, the SQL query took 1.411202ms (vs 6.692714ms before) in median. Still looks a bit slow, but already better.
Even if we decide that we don't want to add one more index to the db, the first commit has value on its own. There's no issue reference because there's no new work, i just moved the useful part of the closed PR here.
