Make report unassignment work fully

[nephila-nacrea]

On an 'all reports' page (e.g. https://staging.fixmystreet.com/reports/Lincolnshire) and a single report page (e.g. https://staging.fixmystreet.com/report/14521), there are 'assign to' dropdowns (available to certain admin users).

Admin can use these to assign reports to members of staff; they should also be able to use the 'unassigned' option to unassign reports. However, unassignment only works if the member of staff who is assigned to a report is the same member of staff who raised the report to begin with.