User can like a specific image

* Add Like model with user_id and image_id
* Add compound index to user_id and image_id
* Add create to like routes
Add like(image) function in User model

Current_user is not available in User model, while
we could use id instead
Add image and like associations

* Shadow image into liked images
Hide like button if user likes the image

Add likes(image) method to determine if user likes the image
Change resources to resource

We only need one path for create and destroy,
and we could do that for group membership as well
User can dislike a specific image
Add an unlike function to User model