Add contributor role skillsets

# Problem

We should consider adding the ability to define "roles" that you'd be looking for in volunteers that serve effectively as buckets of skills.

These roles could then be used to make it easier to quickly grasp how good a fit a new volunteer might be for these roles. They could also be used to quickly assign a bunch of skills to a new task, thinking more about who might be able to complete this.

未指定檔案、測試或進入點。首先在 Ember 應用程式中找到現有的技能、志工和任務表示，然後確定角色將如何與它們關聯。完成標準應包括一個達成共識的角色模型、指派行為，以及用於比對或套用技能集的明確測試。

由索引模型根據 Issue 內容生成。