Use videojs gem instead of vendor/ hacks

This switches to use an existing videojs gem https://github.com/seanbehan/videojs_rails that's nearly identical to the current implementation in /vendor, except:

* includes some more necessary asset url wrapping inside of the video.js file
* doesn't include the Google Analytics tracking-pixel the pre-packaged video.js did

Should make it easier to upgrade without having to re-install our hacks, or fork our own copy if we want to e.g., extract something like YouTube fallback support.