Issue filter bar author or assignee produces errors when not logged in

When using the issue filter bar to select an Author or Assignee, errors will appear if you are not logged in.

error after opening `author:` dropdown	error after selecting `@mikegreiling` as author

For the latter case, looking in my devtools network tab shows a HTTP 401 Unauthorized response to the following query:
https://gitlab.com/api/v4/users.json?search=&per_page=20&username=mikegreiling

I presume there is a permissions issue with the API query we are using.

Suggested remedies:

decide whether this query should be available to logged-out users, and adjust the API if necessary
adjust the frontend code to take this into account and not produce error messages for expected responses