resourceloader: Ensure 'user' loads after 'site' (asynchronously)

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>.

Also:
* 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.

Bug: T32358
Bug: T106736
Bug: T102077
Change-Id: Id599b6be42613529fb7f4dd3273f36ccadb3a09e