FEATURE: Multi-file javascript support for themes (#7526)

You can now add javascript files under `/javascripts/*` in a theme, and they will be loaded as if they were included in core, or a plugin. If you give something the same name as a core/plugin file, it will be overridden. Support file extensions are `.js.es6`, `.hbs` and `.raw.hbs`.

app/helpers/application_helper.rb

@@ -431,6 +431,11 @@ module ApplicationHelper
       &.html_safe
   end
 
+  def theme_js_lookup
+    Theme.lookup_field(theme_ids, :extra_js, nil)
+      &.html_safe
+  end
+
   def discourse_stylesheet_link_tag(name, opts = {})
     if opts.key?(:theme_ids)
       ids = opts[:theme_ids] unless customization_disabled?