AT-20: adapt pagination

every page of feed (except head) is now a full page, i.e. contains exactly pageSize entries. 
We will also paginate use forward/backward urls.

See comments on previous pull request
Pattern matching in the middle of an expression really looks ugly. And specially overkill for a boolean. if / else is more than perfect in such a case.
Can't we simply do?
```scala
val direction = if (forward) "forward" else "backward"
Url(startId.toString) / direction / count.toString
```