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
We don't have a fixed set of code style rules yet, but the general agreement is to use IntelliJ formatting.

Which means that in-fix operators should be surrounded by spaces, for instance:

```scala
minId + 1 to pageSize * 2 map { i =>
  feedStore.push(i.toString)
}
```