Users can edit user reviews they posted

* Extract helper method for displaying modify links for a user review
* Don't show the "Edit" link for reviews created by other users
* Add feature specs for a user editing their own user reviews
* Add unit and controller specs relevant to this PR

Additionally, correct the parent controller class for `Admin::UserReviewsController` so that only admins can access these actions.

https://trello.com/c/CLwkMCBI
Is this the kind of situation you would normally test with a controller spec? (I did.)