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.

Start by reading issue #73 and tracing the existing project listing and activity sources for hack-night check-ins, written updates, and GitHub code check-ins. Define how each source contributes a latest-activity date, then verify that the project view ranks projects by that date and handles projects with no activity.

索引モデルが issue の本文から書いたものです。