This commit fixes the shift+click multi selection in threads. We were not correctly using the manager of the message and would attempt to find messages in the channel instead of the thread.
The `activeThread` was also not correctly set sometimes.
Also adds tests for message selection in threads.
Followup to b94fa3b87a,
which broke the functionality to click on a message
checkbox, hold shift, then click another one, and have
the messages inbetween selected. Add system spec to
catch this.