Allow task reassignment from task edit page

[begedin]

Problem

Users which are allowed to do so should be able to (re)assign tasks from the task edit page. We should use the component extracted in #1172 to achieve this, so this will be marked as blocked until we're ready for that.

Subtasks

• Include component from Extract the task assignment UI from the task card into a component #1172 in project/tasks/task/task.hbs
• Handle onChange action correctly in route/controller
• Fix any styling issues that might occur from using the component in the sidebar instead of the task card.