Change category interacts badly with required tags

[cellio]

A moderator tried to move a question from Q&A to (community) Meta. Meta has required tags, so the move failed. When I tried to reproduce, I realized that the problem is that Meta has required tags and a question coming from Q&A doesn't have that tag. Q&A and Meta use different tag sets, so creating it on the Q&A side is not the right approach for multiple reasons. (Also, I discovered the tag already existed, maybe from someone previously trying that hack, but adding it didn't work.)

We shouldn't fail moves over tags, but we also want to give people a chance to notice and fix tag problems. If Meta didn't have required tags, the original tags in a different tag set would still be wrong.

Let's treat any move as a chance to revisit tags. Show the mover something like:

Original tags: [tagA] [tagB]
Require one of: [discussion] [bug] ...
New tags: (textbox goes here)

For bonus points, if the tag set isn't changing, add a "keep original tags" button so they don't have to be re-entered.

If that's too complicated, then I'd rather see the move succeed with a prompt to edit the post manually. That's better than failing the move. No edit that doesn't fix the tag problem will succeed, so the mod who did the move should clean it up, but if that doesn't happen, other users can still fix it.

Right now, the workaround is for an admin to temporarily disable the required tags, move the post, re-enable the required tags, and edit the post to comply.

[cellio]

The stated workaround no longer works, so I'm raising the priority of this issue. We currently appear to have no way to move Q&A posts to Meta. While this doesn't come up often, when it does it gives new users a bad experience.

[cellio]

New workaround: remove all tags except meta required tag, then move.

[trichoplax]

When working on this, be aware of the following:

1. A failed move from Q&A to Meta (due to not having a Meta required tag) leaves the question in a state where it appears to be in the Q&A category, but any edit (even an edit that makes no changes to tags) results in an error "Not all of this question's tags are in the correct tag set". It seems that the post's category is still Q&A, but somehow its expected tag set is the Meta tag set.
2. The workaround of removing all tags and adding a Meta required tag before trying to move to Meta leaves the Meta required tag showing up in the Tags tab of the Q&A category (with 0 posts).
3. The workaround of removing the required tags from Meta to allow a move can leave a post in the Meta category with a tag from the Q&A tag set (which now shows up in the Tags tab for Meta), and lacking any required Meta tag even after the required tags have been restored for the Meta category.

Hopefully any fix for this bug will also happen to fix these points, but worth double checking before merging.