VE-533 / VE-534: re-style search results, add play button to videos in media item detail page

https://wikia-inc.atlassian.net/browse/VE-533
https://wikia-inc.atlassian.net/browse/VE-534

This commit also addresses two small bugs in `ve.ce.WikiaVideoNode.js` and `ve.ui.WikiaMediaInsertDialog.js`

FYI: ve-label mixin isn't really needed anymore, but since I already spent time writing it (for the previous revision of this ticket), I just left it in there (besides if we want to have that label style somewhere else in the future we will be able to re-use it).