Allow 'No assignee' for external task on single user public boards
Allow 'No assignee' for external task on single user public boards

'No assignee' option is already available in modification but not in
creation.
This patch fixes that by allowing the 'No assignee' option on external
task creation.

kanboard/kanboard,kanboard/kanboard,fguillot/kanboard,BlueTeck/kanboard,renothing/kanboard,kanboard/kanboard,libin/kanboard,fguillot/kanboard,BlueTeck/kanboard,fguillot/kanboard,renothing/kanboard,BlueTeck/kanboard,libin/kanboard,renothing/kanboard