Make use of new usermeta last_activity_tracker

[pkevan]

Since the only current mechanism to track activity is based on log-in, and this is unreliable (see #247), we should make use of the new usermeta to provide an extra mechanism for activity/inactivity.

There are two places to update this:

• On the contributor stats report (Add contributors report, and allow data to be downloaded. #267)
• The outreach for inactivity (which goes out via email)

[pkevan]

noting that the contributor stats part was released. not change on the inactivity, but think this can be closed.