A set of unrelated changes.
Some of them replace checking the thick property with checking another property, e.g. timestamps. This is beneficial because we should handle all the threads with such properties the same.
Some other changes use the existing properties from the specs instead of checking the thick flag.

Depends on D14707

Flow. Checked if the history modal works correctly. Also checked if thick thread messages aren't pruned from the db by creating a thread with multiple messages, forcing the pruning and checking if the messages appear after restart.