CYB-193 Must handle a Video URL submission.

This will allow for processing of video URL as a submission to an assignment.  

*  Handles YouTube (3 formats) and Vimeo URLs
* Introduces an ActiveRecord model, MediaUrl, and so will require a migration

Example VideoUrl output:
```json
{"youtube_id":"qObzgUfCl28","canvas_user_id":4,"assignment_id":3,"author":"Wiggly Otter"},{"vimeo_id":"11099712","canvas_user_id":3,"assignment_id":3,"author":"Canny Raven"}
```