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

spec/requests/forums_controller_spec.rb

@@ -19,24 +19,6 @@ RSpec.describe ForumsController do
     end
   end
 
-  describe "during shutdown" do
-    before(:each) do
-      $shutdown = true
-    end
-    after(:each) do
-      $shutdown = nil
-    end
-
-    it "returns a 500 response" do
-      get "/srv/status"
-      expect(response.status).to eq(500)
-    end
-    it "returns a 200 response when given shutdown_ok" do
-      get "/srv/status?shutdown_ok=1"
-      expect(response.status).to eq(200)
-    end
-  end
-
   describe "cluster parameter" do
     it "returns a 500 response if the cluster is not configured" do
       get "/srv/status?cluster=abc"