When using dropdowns or search in add issues modal should provide feedback

• When using the search box or dropdowns in the add issues modal, should have some type of loading feedback, probably the spinner
• For example, go to this board: https://gitlab.com/gitlab-org/gitlab-ce/boards/257127 and use the add issues modal
• Type something in the search box and hit enter
• It takes a second or two for the modal to refresh
• Same for the dropdowns

added ~12980 label

added ~579196 label

cc @jschatz1 @iamphill @cperessini

added frontend label

This has been like this since the modal was introduced, so i'm going to put it into 9.1. Let me know if you think otherwise @victorwu

assigned to @iamphill

mentioned in commit a051f266

mentioned in merge request !10043 (merged)

changed milestone to %9.1

Added to 9.1.

@jschatz1 : Let me know if this is a problem.

mentioned in issue #29666 (moved)

mentioned in commit 6094b306

closed via merge request !10043 (merged)

mentioned in issue gitlab#8832

mentioned in merge request !14785