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.

先从 issue 中链接的竞争对手概览和活动概览页面开始，然后追踪分配代码和分配标题的渲染方式。完成标准是从显示的代码中移除 `staff-` 前缀，并在活动概览中显示每一种分配代码类型及其对应人员。

由索引模型根据 Issue 内容生成。