Allow serving cable traffic under $SCRIPT_NAME

If deploying Campfire to a path other than the root (for example, using
Kamal's path-based routing) then the Action Cable endpoint needs to be
adjusted to include that path.

Prepending the script name to the cable path means it will be adjusted
to the path in the same way that the application routes are. When script
name is the default, the behaviour is unchanged.

app/helpers/cable_helper.rb

@@ -0,0 +1,7 @@
+module CableHelper
+  def script_aware_action_cable_meta_tag
+    tag.meta \
+      name: "action-cable-url",
+      content: "#{request.script_name}#{ActionCable.server.config.mount_path}"
+  end
+end

app/views/layouts/application.html.erb

@@ -12,6 +12,7 @@
     <%= csrf_meta_tags %>
     <%= csp_meta_tag %>
     <%= current_user_meta_tags %>
+    <%= script_aware_action_cable_meta_tag %>
 
     <%= tag.meta name: "vapid-public-key", content: Rails.configuration.x.vapid.public_key %>
     <%= tag.meta name: "turbo-prefetch", content: "true" %>