API: Clean up skin handling

* Set a more sensible value for the "title" of the page (Special:ApiHelp
  rather than API)
* Avoid calling various skin-related hooks that aren't going to be used.
* Don't override the user in the DerivativeContext, MessageCache doesn't
  pay attention to it anyway for ->parse().

Bug: 72131
Bug: 72174
Change-Id: Idc42ab6526d640dc68fa93f6b4c5524b3d9e2e44
Merge "API: Clean up skin handling"