Use last 5ftF contribution as inactivity criteria

[iandunn]

#27 started sending emails to those who haven't logged in in 3 months. Once #119 and #189 are done, the cron job should be updated to check the last 5ftf contribution, rather than the last login. That'll be much more accurate.

We'll probably need to wait a few months to give that data time to build up, though; or write a script to retroactively populate it.

The last login can still be used as a fallback for teams that don't have enough contribution tracked. See #207

[pkevan]

Picking this up (as it somewhat relates to #248) - it's been agreed initially that we will use any profile activity to count as a marker for being active.

If this throws up any problems we can adjust as necessary.

[pkevan]

https://meta.trac.wordpress.org/changeset/13881/ was added to enable more accurate tracking of profile activity, once that is filled with more data (maybe 1-2 weeks), we can look at using the new user meta to help created a more reliable picture.