Added youtube url adding to interview.

I’ve added in a text input on the interview editing / creation page.
Users can paste in youtube urls there, and it will add them to the
interview. They will be linked to be the ‘watch on youtube’ link on the
interview page.
Merge pull request #6 from cmcnamara87/interview-set-youtube-url

Added youtube url adding to interview. Closes #5 and PT [Finishes #70418038]