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.
Once you've added a unique constraint on (comment_id, user_id) we can implement this using ask for forgiveness logic:

```
like = Like(
    user=user,
    comment=comment
)
db.session.add(like)
try:
    db.session.commit()
except TheExceptionThatGetsThrownWhenUniqueConstraintFails:
   db.session.rollback()
```
```