OutputPage: Ensure setupSkinUserCss() always applies to headElement()

OutputPage: Ensure setupSkinUserCss() always applies to headElement()

Follows-up 80e5b160e0, which had to move this call out of the
headElement() and buildCssLinks() methods as it was no longer
allowed to modify the module queue after it was created.

It was moved to OutputPage::output(), right before Skin::outputPage()
is called, which ends up calling headElement().

The point in time was effectively unchanged for page views.

However for the caller in ApiParse() this meant setupSkinUserCss()
no longer got called at all as it never calls output(), but instead
calls headElement() directly.

Move it to getRlClient(), which is where we set all other
OutputPage-specific things relating to module loading already.

* For page views this has no impact.
* For ApiParse it means headElement(), which calls getRlClient(),
  will once again include skin stylesheets.

Bug: T144301
Change-Id: I5fd4a27fb2d70b98ce9161dc050788d8ac364110