Discord: support announcement channels

[nul800sebastiaan]

A few weeks ago I changed our #news channel into an announcement channel and the channel stopped syncing with Linen. When I changed it back, it didn't re-sync the missed messages but now it is syncing new messages.

Ideally, I would like to have the announcement channel synced properly, but since we didn't really use the specific feature it's fine that it's now back to a chat channel, but a resync should work.