resourceloader: Include global LESS variables in LESS cache key

resourceloader: Include global LESS variables in LESS cache key

This prevents cache churn when the wiki-global LESS variables vary
between wikis because the cache key is used as a "global" instead of
db-local. This is good for the common case, but should still explicitly
vary if the vars differ between wikis.

Bug: T191937
Change-Id: If12fd07a7062792205384150d6f5fd9a83f996cc