SECURITY: Don't escape strip markers when escaping attributes in mw.html

Core strip markers were changed in T110143 to include characters that
are normally encoded in attributes, however we want to pass them through
here so they can be unstripped correctly in the output wikitext.

This fix makes "Strip markers in CSS" parser test pass again.

Bug: T110143
Bug: T135961
Change-Id: I1353931a53c668d8a453dfa2300a99f59fdb01c5