[rotang] Enable notification emails to be sent same day as shift start.

Requested in crbug/928116, sheriffs are used to getting a mail sent at
the beginning of their shifts. Sending it multiple days before caused
some confusion.

- Adds in handling of same day notify e-mail handling.
- Option to turn email generation on/off.
- Add checkbox to the UI.

Bug: 928116
Change-Id: I9f347882222bdae2582e040df8706ad4dcbc8f70
Reviewed-on: https://chromium-review.googlesource.com/c/1453836
Reviewed-by: Scott Lewis-Kelly <slewiskelly@google.com>
Commit-Queue: Ola Karlsson <olakar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#20473}