Toggle to hide message preview for threads

Newly created threads display the last message of that thread as a preview in the text channel they originated from. Oftentimes threads are used for spoilers or sensitive messages that others don't want to see even at a glance.

It would be nice to have toggles to mark threads as spoilers, NSFW or both in order to hide the preview for anyone who isn't currently in that thread, as well as quickly delineate what users can expect even if they aren't paying full attention. A client override to block all previews would also be a nice small QOL feature for extra assurance.