Make active group sync with slack

Right now poor evals has to update the @active group on slack which sounds unfun, and people could just automatically be added to that (and @meetings) when they're made on floor or hit the become active button. This means @active @frosh and @meetings could automatically be cleared when the new year button is hit as well

找出處理 floor 建立、「become active」動作、「new year」動作，以及現有 Slack 群組更新的程式碼。追蹤成員如何指派至 @active、@frosh 和 @meetings，然後確認在每個指定的生命週期事件中，都會發生所要求的新增與清除行為。

由索引模型根據 Issue 內容生成。