Hide "assigned to" bits when there's noone to assign reports to [4hrs]

[chrismytton]

Since the changes to the inspector tool in #3535 we're now showing "Assigned to:" in a couple of places, even when it's not relevant.

On the admin report_edit page, underneath the category dropdown:

On the report page's inspector sidebar:

I think these should only be shown if there are users with the relevant permission.

Related Freshdesk ticket: https://mysocietysupport.freshdesk.com/a/tickets/1579

[dracos]

Also on list pages, anywhere in the country (screenshot is Bridgend), I'm seeing it on every report in the sidebar, it's a bit annoying.

[jacquelinebylau]

Toggl: https://plan.toggl.com/#timeline/317959/tasks/33843711/embed