Remove video fields from non-video formats

* Video url and summary fields aren’t used, they just clutter the
edition form
* There are only 10 published videos, all in the video format
Remove video icons from table list

* Only video formats will have videos, and that’s guessable from the
format type alone
Use input helper classes for video fields

* Avoid using grid wrappers, the admin gem provides helpers for setting
the maximum width of form elements
Merge pull request #282 from alphagov/hide-video-fields

Only show video fields on video formats