View all projects based on recent activity

As a user looking for projects to join, or a staff member making sure that projects are active, it would be good to see projects sorted by recent activity. Activities could include:
- Check-ins at hack nights
- Written updates
- Code check-ins for github-hosted projects (see #73)

Basically, we can be generous for what counts as an 'update' but still rank by date of most recent activity.

首先阅读 issue #73，并追踪现有的项目列表和活动来源，包括 hack-night 签到、书面更新和 GitHub 代码提交。定义每个来源如何贡献最新活动日期，然后验证项目视图是否按该日期对项目进行排序，并处理没有活动的项目。

由索引模型根据 Issue 内容生成。