[bug 899507] Add image upload modal to media page

Clicking "Upload a New Media File" from a media page displays the gallery instead of the upload modal window.

As Ricky explained in the bug we are linking to the media gallery and open the modal window. This is just adding back the piece that was removed in https://github.com/mozilla/kitsune/commit/94fa5fec5935d8f4b96b5a65258acbac5c399ec2