Add moderator role and add pundit policies for admin actions (#5635)

* Add moderator role and add pundit policies for admin actions

* Add rake task for turning user into mod and revoking it again

* Fix handling of unauthorized exception

* Deliver new report e-mails to staff, not just admins

* Add promote/demote to admin UI, hide some actions conditionally

* Fix unused i18n

app/policies/user_policy.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+class UserPolicy < ApplicationPolicy
+  def reset_password?
+    staff? && !record.staff?
+  end
+
+  def disable_2fa?
+    admin? && !record.staff?
+  end
+
+  def confirm?
+    staff? && !record.confirmed?
+  end
+
+  def enable?
+    admin?
+  end
+
+  def disable?
+    admin? && !record.admin?
+  end
+
+  def promote?
+    admin? && promoteable?
+  end
+
+  def demote?
+    admin? && !record.admin? && demoteable?
+  end
+
+  private
+
+  def promoteable?
+    !record.staff? || !record.admin?
+  end
+
+  def demoteable?
+    record.staff?
+  end
+end