Add feature to actively check for updates

[neiljp]

This is a follow-up to #1031. See there for details and more references, but it likely comes down to these elements:

• v1: Add network call to check API updates & warn if out of date, plus note that we call home
• v2: Add "advanced" config feature to disable calling home?

There are nuances as to how to check for updates, when to warn, etc. which we should likely discuss first.