Users can write and view reviews

**Users can write reviews for events:**
* Create migration and model for user reviews
* Add relationships to `Event` and `User` models
* Add `can_review?(event)` helper method to `User` and `Guest` models
* Add validations to `UserReview` model
* Add nested routes for `user_reviews#new` and `user_reviews#create`
* Add form for `user_reviews#new`
* Create `UserReviewsController`
* Only show link to `user_reviews#new` when user has not already reviewed the event

https://trello.com/c/25fI2Eud

**Users can view reviews:**
* Extract partial for showing a single user review
* Small HTML formatting tweaks to `events#show`

https://trello.com/c/R73lNSkr

**Layout changes:**
* Extract partial for displaying form errors
* Display form errors on `user_reviews#new` and `events#new`
* Show flash messages in application layout w/ partial