Expand inactivity notifications to more teams

[iandunn]

#206 will start sending emails to Polyglots and Training contributors (see #27, #190, and Angela's post).

That's just an initial test, though. For it to be effective, we'll need to:

• Evaluate the effects of the Polyglots/Training tests
• Iterate based on feedback
• Publish update post to give team reps a heads up
• Expand it to a few more teams that have contributions tracked, and iterate. Support, Core, Meta, and Community come to mind.
• Expand it to all teams. The teams will need to have contributions tracked in Profiles first, though.

#119 and #189 are related. If a team doesn't have enough activity tracked in Profiles, we may want to fallback to using a simpler criteria, like "has the user logged in to w.org in the past 3 months?"

We could maybe update is_active() to have logic like this:

• if the user is on a team that has lots of activity tracked (core, polyglots, etc), then use the "last contribution" as the factor to determine if they're active
• otherwise use the last login

We could move the team-based logic from prune_unnotifiable_xprofiles() to is_active(), to simplify things. is_active() can handle the different criteria, then everything else can just depend on it instead of having to be aware of that logic.