DEV: Use relative URLs for theme_uploads_local (#17715)

Relative URLs will work just fine for Web Workers, which were the original reason for introducing the `theme_uploads_local` feature

Making them relative will mean that `loadScript()` automatically uses the CDN (when enabled), which is doubly important because our CSP doesn't allow loading theme-javascripts from the host domain when the CDN is enabled.

app/models/javascript_cache.rb

@@ -12,7 +12,7 @@ class JavascriptCache < ActiveRecord::Base
   end
 
   def local_url
-    "#{Discourse.base_url}#{path}"
+    "#{Discourse.base_path}/#{path}"
   end
 
   private