resourceloader: Ensure 'user' loads after 'site' (asynchronously)
Regression from 19a40cd3ad which made the 'site' module load asynchronously,
but the 'user' module was still loaded synchronously which meant it ran before
the site module finished.
Full test script at https://gist.github.com/Krinkle/5db1d237da241b243485.
- This changes the 'user' module to load asynchronously.
- Similar to 19a40cd3ad for site module, this makes the styles for the user module load twice. Harmless but doesn't look pretty internally.
- Remove the obsolete XXX-comment from 0b5389d98d (r56770).
- Add comment documenting the fact that the 'excludepages' feature can cause User/common.js and User/vector.js to be mis-ordered when the user previews common.js edits. This has always been the case (since 2009) and is merely being documented here.