Update UserDatatable for ajax-datatables-rails 1

[AndrewKvalheim]

Checklist

• I have read the Contribution & Best practices Guide.
• My branch is up-to-date with the upstream master branch.
• The tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works (if appropriate).
• I have added necessary documentation (if appropriate).

Short description of what this resolves

UserDatatable hasn’t been migrated to ajax-datatables-rails v1 and fails to load data with NotImplementedError:

1) UserDatatable outputs recordsTotal
   Failure/Error: filter_records(fetch_records).unscope(:group, :select).count(:all)

   NotImplementedError:
     NotImplementedError
   # ./app/datatables/user_datatable.rb:59:in `records_filtered_count'
   # ./spec/datatables/user_datatable_spec.rb:95:in `block (3 levels) in <top (required)>'
   # ./spec/datatables/user_datatable_spec.rb:98:in `block (3 levels) in <top (required)>'

Changes proposed in this pull request

Update UserDatatable as RegistrationDatatable was in 81853d1.