Various fixes related to thread support

[ChrisLovering]

This fixes a few bugs around threads namely:

1. Ignore threads within forum channels. These aren't supported by d.py so we want to avoid tracebacks.
2. Listen for thread create and unarchive events explicitly
   • Discord.py only firsts thread_update events for threads in the cache, this isn't the case for threads being created, and threads being unarchived, so there events were previously being missed from the channel sync.
3. Only sync archive status on startup
   • Since we now sync archive flags during bot runtime, there is no need to sync this every time we sync threads.

[jchristgit]

Trust.