abdmail_mailer: refactor, format whole string

Instead of using concetenation, compose the email message string as one
whole string and use format to insert strings, fix the indentation at
run-time using textwrap.dedent().

This seems clearer and certainly less fiddly than concatenation.

Test Plan:
$ ./precommit.sh
examine emails from
$ ./testbed/arcyd/acceptance_tests.sh
abdmail_mailer: better 'unknown email' message

Improve the email that users and admins receive when someone has pushed
a review with an email unknown to Phabricator.

Make suggestions for what to click or type to resolve the issue.

Test Plan:
$ ./precommit.sh
Watch email output from
$ ./testbed/arcyd/acceptance_tests.sh