Users can leave groups

* Add link to leave a group to the groups show page
* Add route for leaving a group
* Implement group_memberships#destroy
* Add method in user model for leaving a group
Extract method for determining if a user belongs to a group
Show flash messages when joining or leaving a group
Allow users to like and unlike images

* Create migration and model for likes
* Add helper methods to user model for liking and unliking an image
* Update image and user models to reference likes
* Add routes for creating and deleting likes
* Update images show view to display number of likes
* Update images show view to display like/unlike links