It might happen that some User records have no associated primary emails.
In which case we don't ever want to send them a digest.
Also added a new "user_email_no_email" skipped email log to ensure these cases
are properly handled and surfaced.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Régis Hanol
Martin Brennan
Gerhard Schlager
Sam Saffron
David Taylor
Guo Xiang Tan