Full YT links in Mercury

PT Story: https://www.pivotaltracker.com/story/show/78252474

- created `vendor/assets/javascripts/mercury_overrides.js` with a new validator for `Mercury.modalHandlers.insertMedia`-function
- added new scenarios to `documents.feature`

- used `git rebase -i develop` and ended up with some commits by @betasve and @christophermlne so... WTF? 

The solution is taken from `https://github.com/slonia/mercury/blob/c898a94e28265d78ecb5e4502621d2a4a154a4c1/app/assets/javascripts/mercury/modals/insertmedia.js.coffee` and translated to plain javascript (we are not using coffee script in WSO). We currently do not have any jasmine tests for the code in `mercury_overrides.js`
“is invalid” is a little too generic, how hard would it be to make it more specific to this case?