Implement post liking with a "Like" model instead of HABTM

* Create Like model
* Create migrations to drop existing likes table and replace it with one that corresponds to new likes model
* Create likes helper to make creating like buttons easier (based off of subscribe button)
* Modify ability.rb to allow users to create Likes
* Modify User and Post models to have many likes
* Create likes index page
* Modify single post partial to use like_button helper instead of posts/like partial
* Add likes resource to routes