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.

app/controllers/application_controller.rb

@@ -321,7 +321,11 @@ class ApplicationController < ActionController::Base
       with_resolved_locale(check_current_user: false) do
         # Include error in HTML format for topics#show.
         if (request.params[:controller] == 'topics' && request.params[:action] == 'show') || (request.params[:controller] == 'categories' && request.params[:action] == 'find_by_slug')
-          opts[:extras] = { html: build_not_found_page(error_page_opts), group: error_page_opts[:group] }
+          opts[:extras] = {
+            title: I18n.t('page_not_found.page_title'),
+            html: build_not_found_page(error_page_opts),
+            group: error_page_opts[:group]
+          }
         end
       end
 
@@ -896,6 +900,7 @@ class ApplicationController < ActionController::Base
     end
 
     @container_class = "wrap not-found-container"
+    @page_title = I18n.t("page_not_found.page_title")
     @title = opts[:title] || I18n.t("page_not_found.title")
     @group = opts[:group]
     @hide_search = true if SiteSetting.login_required