Ability to 'like' a post

[GautamGupta]

Backend:

• Create a new table to store post id, user id, like timestamp (created_at)
• Add a new column to posts table to store a cached value of the # likes
• Add backend methods to like and unlike topics performing the 2 db operations above
• Add backend method to know if the user currently likes the topic.
• Add a new route and controller logic to like the topic for the current user (this will be used by frontend)
• Add tests

Frontend:

• Add javascript logic to show the like/unlike link and perform the XHR request
• Handle errors