Quick Tags: Work correctly on non-NPF posts

[marcustyphoon]

Description

This adds an alternative API endpoint call to Quick Tags on posts that we cannot ensure the NPF edit endpoint will work on. It uses the mass post editor endpoint, which should work on both legacy and NPF posts.

For discussion about which posts are NPF-compatible, see discord.com/channels/104051306309644288/327155782934200320/1074286553733533738.

Resolves #940 (most likely reason). Resolves #965. Resolves #1224.

Testing steps

[marcustyphoon]

- [ ] see if the mega editor endpoint changes tipping

[marcustyphoon]

This next commit addresses the above comment and #1046, though I'm not sure if this is the ideal way to do so (vs, for example, making the mega editor error wrapped in { body: contentsGoHere }.

[marcustyphoon]

Besides the virtual scroller stuff, this is IMO a good priority, as the virtual scroller broke legacy XKit's Quick Tags and that seems like a great excuse to have people switch ahem, reason to ensure we have a good alternative.

[marcustyphoon]

Now that I think about it, it's probably better not to mention legacy posts in the copy at all, considering that there is some ambiguity there.

[AprilSylph]

Eh, the copy is accurate. If we're displaying that message then it's definitely a legacy post, even if it doesn't display for all legacy types.