Retry failed email deliveries

[raboof]

Sometimes, such as at https://release-test.apache.org/vote/pekko/2.0.0-M1, sending the vote email will fail. In that case it might be nice to retry automatically and/or give the user an option to retry explicitly.

[sbp]

Depends on #209. The root cause is that we haven't chosen an email system yet because we haven't chosen a way to do mocking and (some level of) production testing. There are a few possible architectures but we haven't discussed them; the choice of architecture will guide our specific mail sender choice. The current implementation is a simple send direct to the relay, and this code will go away.