Point Action Cable at the slugged cable endpoint

I proposed this upstream in https://github.com/rails/rails/pull/55250,
but it's not clear if the presence of script_name should _always_
prefix generated URLs, so for now it's local to Fizzy.

app/helpers/tenanting_helper.rb

@@ -0,0 +1,7 @@
+module TenantingHelper
+  def tenanted_action_cable_meta_tag
+    tag "meta",
+        name: "action-cable-url",
+        content: [ request.script_name, ActionCable.server.config.mount_path ].join("/")
+  end
+end

app/views/layouts/shared/_head.html.erb

@@ -15,6 +15,7 @@
   <%= stylesheet_link_tag :app, "data-turbo-track": "reload" %>
   <%= javascript_importmap_tags %>
 
+  <%= tenanted_action_cable_meta_tag %>
   <%= yield :head %>
 
   <link rel="manifest" href="/manifest.json">