Convert admin/registrations datatable to AJAX-backed

Supercedes https://github.com/openSUSE/osem/pull/2031 .

Note:

* "Questions" modals have been dropped from this view; questions are no
  longer asked on registration, since Surveys(PR#1100)  were added.

app/datatables/registration_datatable.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+class RegistrationDatatable < AjaxDatatablesRails::Base
+  def_delegator :@view, :edit_admin_conference_registration_path
+
+  def view_columns
+    @view_columns ||= {
+      id:                       { source: 'Registration.id', cond: :eq },
+      name:                     { source: 'User.name' },
+      roles:                    { source: 'Role.name' },
+      email:                    { source: 'User.email' },
+      accepted_code_of_conduct: { source: 'Registration.accepted_code_of_conduct', searchable: false },
+      arrival:                  { source: 'Registration.arrival', searchable: false },
+      departure:                { source: 'Registration.departure', searchable: false },
+      actions:                  { source: 'Registration.id', searchable: false, orderable: false }
+    }
+  end
+
+  private
+
+  def conference
+    @conference ||= options[:conference]
+  end
+
+  def conference_role_titles(record)
+    record.roles.collect do |role|
+      role.name.titleize if role.resource == conference
+    end.compact
+  end
+
+  def data
+    records.map do |record|
+      {
+        id:                       record.id,
+        name:                     record.user.name,
+        roles:                    conference_role_titles(record.user),
+        email:                    record.email,
+        accepted_code_of_conduct: !!record.accepted_code_of_conduct, # rubocop:disable Style/DoubleNegation
+        arrival:                  record.arrival&.utc,
+        departure:                record.departure&.utc,
+        questions:                {},
+        edit_url:                 edit_admin_conference_registration_path(conference, record),
+        DT_RowId:                 record.id
+      }
+    end
+  end
+
+  def get_raw_records # rubocop:disable Naming/AccessorMethodName
+    conference.registrations.includes(user: :roles).references(:users, :roles).distinct
+  end
+
+  # override upstream santitation, which converts everything to strings
+  def sanitize(records)
+    records
+  end
+end