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>
formatter: include 'cc' in RSS <channel>'s <link>

* Adds collection link to channel if the current collection is defined.
  (addresses #2013)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
search: user-preffered output format

* Introduces a new parameter `of` for personal
  default output format.  (closes #1587)

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
legacy: oaiharvest admin import fix

* Improves PEP8 and PEP257. (PR #2194)

* Temporary fixes wrong imports. (addresses #2188)

Co-authored-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
legacy: websearch circular import removal

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
legacy: bibindex admin interface fix

* Fixes bibindex admin interface. (closes #2190)

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
legacy: legacy admin interfaces addition

* Adds all the admin interfaces to the `/admin` page.

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>