Vid 1551

Don't merge just yet. There's still some more work to do, I just created this PR to review progress. 

Removed a bunch of unused code related to the old thumbnails. The main things that were removed were:
- Anything that used the css class "Wikia-video-play-button" 
- WikiaFileHelper::videoPlayButtonOverlay and all the function only it called
- Anywhere that 'useTemplate' was set in Oasis since Oasis is fully migrated. WikiaMobile is still migrating. 
- Removed 'isVideoThumb' from LatestPhotos functions as they will never include video thumbs. 

Note: I had to do a slightly hacky refactor of the WikiaPhotoGallery slider editor preview so that it could use the new style of play button (it was the only place I found that was still using the old style), but that extension will go away soon anyway. 

This is part of the slider preview hack