Give mods a way to revoke comment ability

[cellio]

When we built abilities, we bundled "comment anywhere" in with Participate Everywhere. This makes sense; there's no separate metric that makes sense for comments the way post score, edit score, and flag score do for other abilities.

But... sometimes somebody who's otherwise productive just needs to Stop Commenting for a while. Somewhere Else, mods struggled with what to do in this case, as the only available tool was suspension and that was a big stick.

With abilities, moderators could suspend the Participate Everywhere ability, but that blocks some posting and most voting, and that is also a big stick.

Can we give moderators a way to specifically suspend commenting for a period of time? This could be either through a "Comment Everywhere" pseudo-ability granted along with Participate Everywhere, or it could be something that's not ability-based but is part of the suspension logic (suspend just that vs everything). I think the pseudo-ability fits with our design better, except that its "how to earn it" page will be a little different since it won't have its own trackers. Or maybe we make it a real ability and it's a clone of Participate Everywhere? @luap42 should definitely weigh in on this before anybody starts work, but I'm filing this now so I don't lose track of it again (I already have once or twice).