1. Changes default email format to llll (eg: Tue, May 8, 2018 2:00 AM)
from 2018-05-08T00:00:00Z UTC which is not nice on the eyes
2. Adds `discourse_local_dates_email_timezone` which allows configuring
default timezone in emails
3. Improved help text on site settings (format / timezone)
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Co-authored-by: Gary Pendergast <gary@pento.net>
