Prevent posting new root messages

[edemaine]

In class settings for example, I like to have control over root-level messages, so want to prevent some users from doing so. But I still want them to be able to reply to existing root-level messages. I can see two possible permission models:

1. There's another user permission after Read/Post/Edit, like Root Post. This seems like a lot of complexity for a small feature, but is the most flexible.
2. There's a group-level flag called something like locked meaning posts aren't possible except by superusers. Plausibly, threads could be locked too, though this is going to be implemented differently anyway (as group and thread views and databases are different), and I also can't think of an application of this. Locked threads would I guess be similar to "private replies only" threads, so turning that around, we could imagine having a group-level flag of "public root post only, private root post only, or both enabled", for symmetry with threads...

Natural terminology for this is "protected group", by analogy to protected messages (#507).

[jbosboom]

Locking a thread may be useful for making polls with the voting/reaction system: make one submessage per poll option, then lock the thread. Users can vote on the submessages, but can't post "+1 I really like this one!"/"Me too!" messages.

If you're polling often you'd want an actual poll feature, of course, and maybe you don't want to poll at all.