Establish criteria for sending emails to all users

[mattbk]

Many people didn't learn about the attempted 1.0 refund. I'm sure there are other very important things that people haven't learned about this summer, either.

Are there rules that can be applied to determine how important a particular thing is and whether it is email-worthy?

[chadwhitacre]

For additional background, see #235.

[chadwhitacre]

With gratipay/gratipay.com#3843 we'll have a way for users to unsubscribe from "News & Announcements." With that in place we could potentially adopt a more frequent schedule—should we announce every blog post?

[chadwhitacre]

Also, an announcement may not need to go to all users (in the case of the 1.0 refund we only needed to notify people with a balance).