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
Due to the complexity of this class, I think we should add more modeling. For instance, it's not clear what the following tuple means: (Option[Long], List[(Long, Entry[E])], Option[Long])
We could create a local case class for that. Also for the List type (Long, Entry[E]).

If we work with tuples, we have to constantly think from where they come from and how we must disassemble it. Moreover we end up using ._1, ._2, ._3, etc which is much less readable than x.previous, x.entries, x.next