Make "Auto-Submitted: auto-generated" header optional with a settings flag

Some mailing lists filter out emails containing the "Auto-Submitted:
auto-generated" header. Making this header optional allows those who use such
mailing lists to receive Review Board emails.

Testing Done:
* Manually enabled and disabled the header settings flag in E-Mail Settings.
* Ran unit tests.

Bugs closed: 2646

Reviewed at https://reviews.reviewboard.org/r/5276/