Refs dropdown is now loaded async
## What does this MR do?
The refs dropdown is loaded async so not to block the page.
## What are the relevant issue numbers?
Part of #18202
## Screenshots (if relevant)
![Screen_Shot_2016-06-07_at_14.41.21](/uploads/8fea12655f96fe7f7008a32677bff037/Screen_Shot_2016-06-07_at_14.41.21.png)
See merge request !4508