Add last_activity_on for groups

Problem to solve

It's hard to figure out whether or not a group is active in GitLab. We have last_activity_on for projects that gets updated on project activity, but we don't have anything similar for groups.

Proposal

Introduce a last_activity_on attribute for groups.

• If a project in the group has any activity, update the group's last_activity_on as well.

A group's last_activity_on should inherit upwards into parent groups. E.g. if someone pushes to a project in a subgroup, the subgroup's parent group should get an updated last_activity_on date as well.

What does success look like, and how can we measure that?

• We're able to easily understand whether or not a group is still active. This is especially important for GitLab.com.