Deprecate unconditional loading of mediawiki.ui.button on all pages

Deprecate unconditional loading of mediawiki.ui.button on all pages

Formally deprecate the unconditional loading of mediawiki.ui.button on all
pages. During the deprecation period, the styles will be loaded on pages which
contain 'mw-ui-button' in the page HTML. Starting with 1.28, the styles will
only be loaded if explicitly depended upon.

The rationale for the patch was expressed by Spage in a comment on PS4 of
I835f3eb937: "Wikis have too many templates like
[[commons:Template:Clickable_button]] dependent on jquery.ui.button that don't
work if JavaScript isn't enabled.  mediawiki.ui.button is a lighter-weight
piece of CSS that we own and can improve, and transitioning wiki content to it
requires that we provide it." Mattflaschen commented that the added payload was
"justifiable if we announce this widely and make an effort to get wikis to use this."

A year and a half later, [[commons:Template:Clickable_button]] is still not
using mediawiki.ui.button, and usage elsewhere is not sufficiently widespread
to merit loading this stylesheet in the critical path.

Bug: T98395
Change-Id: I290e1db422b7c7b9c1a026aae22e174580cefae9