Fixes T5492. I figured this would be easier to just fix than write a guide for; it actually took me an hour, but I spent like 75% of that futzing with my editor.
- The Move controller currently accepts either a slug or an ID. I can't find any callsites which pass a slug, and this doesn't make sense. Pretty sure this was copy/pasted from Edit or something. Only accept IDs.
- Slightly modernize the Move controller (newDialog(), handleRequest(), $viewer).
- When the user enters a bad slug, warn them that we're going to fix it for them and let them accept or reject the changes.
- Don't prefill the edit note (this feels inconsistent/unusual).
- On the form, label the input "Path" instead of "URI".
- Show the old path, to help remind the user what the input should look like.
- When a user tries to do a no-op move, show a more tailored message.
- When the user tries to do an overwriting move, explain how they can fix it.
- When normalizing a slug like /question/???/mark/, make it normalize to /question/_/mark.