Do not allow text input in dropdown while loading

## What does this MR do?
It moves the focus event of the text input of a filterable dropdown to after loading of options is complete. After the fix, the user cannot edit the while it's greyed out and loading.
## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?
It fixes the bug in https://gitlab.com/gitlab-org/gitlab-ce/issues/23496.
## Screenshots (if relevant)
![https://gitlab.com/gitlab-org/gitlab-ce/uploads/a7bc2f0228fcde5a85cccb333b52f0e3/2016-10-18_12.00.54.gif](https://gitlab.com/gitlab-org/gitlab-ce/uploads/a7bc2f0228fcde5a85cccb333b52f0e3/2016-10-18_12.00.54.gif)

## What are the relevant issue numbers?
Fixes #23496

See merge request !7054