Search dropdown in web UI should have better feedback on slow links

Minor weirdness: on very slow links, it's hard to know if the web client is trying to recover a list of users or whether the "No results" message is left over from a previous search. Adding a spinner or other feedback after some timeout might be helpful, in it's absence, one knows the search yielded the same set of nothing as one's previous search.

Start by locating the web UI search dropdown and reproducing the behavior on a slow link. Trace how loading and “No results” states are shown; done means users can distinguish an in-progress user search from an empty result, with the behavior covered by the relevant UI test if one exists.

Written by the indexing model from the issue text.