Fix dropdown position when loading remote data

What does this MR do?

When a dropdown that loads data async is opened and the dropdown opens upward, the dropdown div has a CSS translate based on the height of the div. But this is not recomputed after the data is loaded and the dropdown populated. A recompute is only triggered on window resize / scroll.

Causing the behaviour seen in the labels select of the issue boards sidebar when window height is small:

Screen_Recording_2019-05-22_at_10.26.42_AM

This MR forces a recompute of the position after the AJAX data is loaded.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #61639 (closed)