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 (https://github.com/WordPress/five-for-the-future/pull/267)
• The outreach for inactivity (which goes out via email)