restful: pagination fixes

* Refactors the `restful_pagination` file.  (closes #2102)

* Simplifies `link_header` method when concatenating links into
  one string.

* Renames file `restful_pagination` to `pagination`.
  (addresses #1724)

* Improves PEP8 and PEP257.  (PR #2087)

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>