Generalize ResourceLoader 'excludepage' functionality

Generalize ResourceLoader 'excludepage' functionality

There has long been a hack for previewing edits to user JS/CSS, where
OutputPage would pass an 'excludepage' parameter to
ResourceLoaderUserModule to tell it not to load one particular page and
would instead embed that page statically. That's nice, but there are
other places where we could use the same thing.

This patch generalizes it:
* DerivativeResourceLoaderContext may now contain a callback for mapping
  titles to replacement Content objects.
* ResourceLoaderWikiModule::getContent() uses the overrides, and
  requests embedding when they're used. All subclasses in Gerrit should
  pick it up automatically.
* OutputPage gains methods for callers to add to the override mapping,
  which it passes on to RL. It loses a bunch of the special casing it
  had for the 'user' and 'user.styles' modules.
* EditPage sets the overrides on OutputPage when doing the preview, as
  does ApiParse for prop=headhtml. TemplateSandbox does too in I83fa0856.
* OutputPage::userCanPreview() gets less specific to editing user CSS
  and JS, since RL now handles the embedding based on the actual
  modules' dependencies and EditPage only requests it on preview.

ApiParse also gets a new hook to support TemplateSandbox's API
integration (used in I83fa0856).

Bug: T112474
Change-Id: Ib9d2ce42931c1de8372e231314a1f672d7e2ac0e