Give proper support for different types of assignment codes

On the competitor's own overview page, all types of possible assignments are written https://www.competitiongroups.com/competitions/RoskildeForAlle2022/persons/1

However, it lacks two kinds of (somewhat easy) supports.
1: For assignment codes, if there is a 'staff-' prefix, this should not be displayed. If the assignment code is listed as 'staff-runningJudge' it would be displayed as exactly that, where it would make sense to simply write 'runningJudge'.

2: On the activity overview (https://www.competitiongroups.com/competitions/RoskildeForAlle2022/activities/20), the other types of assignment codes are not listed. It should be as simple as making the same kind of header as for all the other assignments and list the people under it.

先從 issue 中連結的競爭對手總覽和活動總覽頁面開始，接著追蹤指派代碼和指派標題的呈現方式。完成標準是從顯示的代碼中移除 `staff-` 前綴，並在活動總覽中顯示每一種指派代碼類型及其對應人員。

由索引模型根據 Issue 內容生成。