DEV: Avoid $ globals (#15453)

Also:
* Remove an unused method (#fill_email)
* Replace a method that was used just once (#generate_username) with `SecureRandom.alphanumeric`
* Remove an obsolete dev puma `tmp/restart` file logic

app/controllers/forums_controller.rb

@@ -17,11 +17,6 @@ class ForumsController < ActionController::Base
       end
     end
 
-    if $shutdown # rubocop:disable Style/GlobalVars
-      render plain: "shutting down", status: (params[:shutdown_ok] ? 200 : 500)
-    else
-      render plain: "ok"
-    end
+    render plain: "ok"
   end
-
 end