R1:120a7d916447

Improve Phriction page move dialog

Summary:
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).