Only load gallery styling rules when galleries are on the page

Only load gallery styling rules when galleries are on the page

* Double load styling rules in legacy modules so we have time for
cached pages to catch up
** Double loading styles is acceptable for 30 days. There is no better way.
* Load gallery css when gallery tag invoked.

To test:
* Visit a page with a gallery tag and purge it, note styles are present.
* Visit a page without a gallery tag and purge it, note styles are not present

Bug: T98878
Change-Id: Ib1aef04dc4fece78e6615386ecaef6a9f368f49e