Automatically notify inactive contributors/companies, to encourage them to either start contributing again, or update their pledge data to be more accurate.

We should finish https://github.com/WordPress/five-for-the-future/milestone/8 before starting this, so that the activity data is accurate.

Tracked in #169.
See https://make.wordpress.org/project/2021/09/10/suggested-iterations-for-the-five-for-the-future-program-and-tool/ and https://make.wordpress.org/project/2022/03/16/proposal-to-remove-spam-dormant-five-for-the-future-pledges/