RelatedPages: add caching in get() method
RelatedPagesApiController: use WikiaResponse const instead
CategoryRecommendation: get data from RelatedPages
VideoProvider: check $wgEnableVideosModuleExt
Use long PHP tags
RelatedPages: use per-wiki memcache key
Merge branch 'CON-1797' into CON-2086-related-pages-provider

Conflicts:
	includes/wikia/api/Recommendations/DataProviders/Video.class.php
Merge pull request #5328 from Wikia/CON-2086-related-pages-provider

CON-2086: Related Pages provider
Merge remote-tracking branch 'origin/CON-1797' into CON-2088