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