Responsive  class for YT videos

PT Chore https://www.pivotaltracker.com/story/show/79654966

- added `video-container` css class 
- wrapped yt players with a div with  `video-container` on:
   - scrums timeline
   - user/show
   - project/show `_video_list.html.erb` 
- Added a wrapper to mercury editor to be added to `<iframe>` when inserting yt file