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/account_policy.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+class AccountPolicy < ApplicationPolicy
+  def index?
+    staff?
+  end
+
+  def show?
+    staff?
+  end
+
+  def suspend?
+    staff? && !record.user&.staff?
+  end
+
+  def unsuspend?
+    staff?
+  end
+
+  def silence?
+    staff? && !record.user&.staff?
+  end
+
+  def unsilence?
+    staff?
+  end
+
+  def redownload?
+    admin?
+  end
+
+  def subscribe?
+    admin?
+  end
+
+  def unsubscribe?
+    admin?
+  end
+
+  def memorialize?
+    admin? && !record.user&.admin?
+  end
+end