Fix Modal for Video Fields.

Probably missed from upgrading from Bootstrap 2 to 3. According to [Bootply](http://webcache.googleusercontent.com/search?q=cache:http://www.bootply.com/bootstrap-3-migration-guide)
 * remove `.hide` from the `.modal` (it's now hidden by default)
 * wrap `.modal-header` `.modal-body` `.modal-footer` inside `.modal-content`
 * wrap `.modal-content` inside `.modal-dialog`

Did a quick `grep` and found no other faulty `modal`s.