Video sprint20 image page redesign

This is the implementation for ImagePage redesign as stated here:
https://wikia-inc.atlassian.net/browse/VID-87

This changeset has the following:
* Introduced WikiaImagePage, which inherits includes/ImagePage.php
* All file page requests now goto either WikiaImagePage or WikiaVideoPage
* There is a temporary variable called $wgEnableVideoPageRedesign to stage rollouts.  It's an unfortunate naming, but VideoPage design is already out, and it's probably not worth dealing with yet another global.
* Renamed some resources from VideoPage to WikiaFilePage
* It also includes a sub case of VID-87, VID-97 https://wikia-inc.atlassian.net/browse/VID-97.  There is a "replace" option if file page.