As an admin, I can edit and delete user reviews

**Admins can edit user reviews**
* Add edit and update routes for `admin/user_reviews`
* Create `Admin::UserReviewsController`
* Extract user reviews form fields into a partial
* Update `user_reviews#new` view to use partial
* Add `admin/user_reviews#edit` view
* Add link to `user_review` partial to display edit link for admin users

**Admins can delete user reviews**
* Add destroy route for `admin/user_reviews`
* Add destroy action to `Admin::UserReviewsController`
* Add delete link to `user_review` partial for admins

https://trello.com/c/3D2jEhlC