Add sorting criteria support for user resource requests

[dan0x09]

Pagination has been implemented on the backend side for user resources for a while now, but has not been made of use on the frontend side up til now. Currently, the User List component in the frontend is being updated to support pagination.
In an issue in the frontend repository, I suggested enhancing the User List with a sorting feature, s.th. the user can decide whether they want the list to be sorted by Id or username, for example. Now that we will be using pagination, sorting can no longer be handled by the frontend alone, since only a subset of the users is requested at a time. Therefore, we should add support for sorting as specified by JSON API: https://jsonapi.org/format/#fetching-sorting