variable days back for bin/mail volume

Summary:
Email is so exciting I can't wait 30 days for initial results.

ref T9161

Test Plan:

• ./bin/mail volume --days 60 took longer and gave plausibly larger results.
• ./bin/mail volume --days 0 quickly told me no mail had been sent.
• ./bin/mail volume Said it was still looking 30 days back.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T9161

Differential Revision: https://secure.phabricator.com/D13901