Feature/liking a comment

This feature is built on top of the #9 and #12 PR:s, which should be merged first.
Adds like model, api for liking and less styles for liking.
Lets make this operation atomic:
```
rows = Like.query.filter(User.id == user_id, Comment.id == comment_id).delete()
if not rows:
    return (
        jsonify({'error': 'No comment was found.'}),
        400
    )
db.session.commit()
return '', 204
```