We've had requests for allowing users to be opted out of Review Bot

e-mail notifications, so that only the review request owner gets

notifications from Review Bot reviews.
This change adds a tool setting to allow for this. When enabled, e-mail

notifications of Review Bot reviews will only be sent to the owner of

the review request. This makes use of our existing to_owner_only flag

for reviews, plumbing it through to Review Bot.

Used the setting on a tool, saw that the email notification was only

  sent to the review request owner when the setting was enabled. And

  that it went to the reviewers as well when the setting was disabled.
Ran unit tests.