FIX: Ensure theme_uploads_local only has one / at beginning (#17719)

Followup to c7dfb1c549

app/models/javascript_cache.rb

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