FEATURE: Add page title to 404 pages (#16846)

The title had to be added both on the 404 page generated by the server
side, displayed when the user reaches a bad page directly and the 404
page rendered by Ember when a user reaches a missing topic while
navigating the forum.

spec/requests/exceptions_controller_spec.rb

@@ -7,6 +7,11 @@ RSpec.describe ExceptionsController do
 
       expect(response.status).to eq(404)
 
+      expect(response.body).to have_tag(
+        "title",
+        text: "#{I18n.t("page_not_found.page_title")} - #{SiteSetting.title}"
+      )
+
       expect(response.body).to have_tag(
         "img",
         with: {