Allow task reassignment from task edit page

# 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 #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.

此 issue 受 #1172 中的元件阻擋。該元件準備好後，從 project/tasks/task/task.hbs 開始，接著追蹤 task 的路由/控制器以處理其 onChange 動作。完成表示元件已包含在 task 編輯頁面中、重新指派對獲准的使用者有效，並在需要時修正側邊欄樣式。

由索引模型根據 Issue 內容生成。