DEV: allow themes to render their own custom homepage (#26291)

This PR adds a theme modifier and route so that custom themes can opt to show their own homepage. See PR description for example usage.

config/locales/client.en.yml

@@ -1672,6 +1672,9 @@ en:
         default: "(default)"
         any: "any"
 
+      homepage:
+        default: "(default)"
+
       password_confirmation:
         title: "Password Again"
 
@@ -4183,6 +4186,9 @@ en:
         this_week: "Week"
         today: "Today"
 
+    custom_homepage:
+      admin_message: 'One of your themes has enabled the "custom_homepage" modifier but it does not output anything in the [custom-homepage] connector. (This message is only shown to site administrators.)'
+
     browser_update: 'Unfortunately, <a href="https://www.discourse.org/faq/#browser">your browser is unsupported</a>. Please <a href="https://browsehappy.com">switch to a supported browser</a> to view rich content, log in and reply.'
 
     permission_types:

config/routes.rb

@@ -1586,6 +1586,8 @@ Discourse::Application.routes.draw do
          constraints: HomePageConstraint.new("finish_installation"),
          as: "installation_redirect"
 
+    root to: "custom#index", constraints: HomePageConstraint.new("custom"), as: "custom_index"
+
     get "/user-api-key/new" => "user_api_keys#new"
     post "/user-api-key" => "user_api_keys#create"
     post "/user-api-key/revoke" => "user_api_keys#revoke"