22204: Fix sharing dialog loading

22204: Fix sharing dialog loading

* Split user/group queries so we don't ask for groups from the user
endpoint and vice versa
* Make each query conditional on whether any permissions to users or
groups were actually returned
* Send user/group queries in parallel
* Wrap the whole thing in try/finally to make sure it stops the
loading spinner if there's an error

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>