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.
Actually, I think it might make more sense to reverse the logic for liking and disliking, because probably shouldn't be able to list who else have liked a comment, just how many people have liked:

Liking:

```http
POST /users/me/likes
```
```json
{
  "links": {
    "comment": 123
  }
}
```

```http
DELETE /users/me/likes/:comment_id
```