Add feature specs for admin creating media review; refactor

* Add feature specs for admin creating a media review and seeing the
  analyzed review data (including analyzed text) on success and the
  review itself on the event page.
* Use VCR to record and replay web service calls for these specs
* Extract `MediaReviewHelpers` module with reusable helper methods

Additionally, refactor the `admin/media_reviews#create` view, using the
media review partial to display information about the created review and
remove unneeded links