Users can flag posts as spam

* Create model and migration for spam flags
* Define relationships between Post, SpamFlag, and User models
* Create helper method for User model to flag a post as spam
* Create helper method for Post to determine if it's flagged by a user
* Add link to _post partial to flag a post as spam
* Add route and controller for spam_flags#create

https://trello.com/c/9V568pes

@ikalu @wowenstine @twentee7 
Unnecessary instantiation

```ruby
spam_flags.exists?(user: user)
```

http://api.rubyonrails.org/classes/ActiveRecord/FinderMethods.html#method-i-exists-3F