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/custom_emoji_policy.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+class CustomEmojiPolicy < ApplicationPolicy
+  def index?
+    staff?
+  end
+
+  def create?
+    admin?
+  end
+
+  def update?
+    admin?
+  end
+
+  def copy?
+    admin?
+  end
+
+  def enable?
+    staff?
+  end
+
+  def disable?
+    staff?
+  end
+
+  def destroy?
+    admin?
+  end
+end