Review Board tries to send e-mails on behalf of users by spoofing their

e-mail address, in order to offer better comment threads. While it tries

to be smart about when to do this (by checking DNS records to see whether

it can safely spoof the e-mail address), it's sometimes ideal to simply

turn off this behavior and send from the server's configured e-mail

address instead.
This change adds an option to force the server's e-mail address to be

used for all e-mails. When enabled, e-mails will still appear to come

from the user, but only by name (using "<Name> via Review Board

server@email" as the From address).
In order for this change to correctly send e-mails using the "<Name> via

Review Board server@email" format, the mail message in djblets must be

updated to send e-mails using that format when email_smart_spoofing is

set to False.

The notifications tests were updated to check that the email used is

settings.DEFAULT_FROM_EMAIL instead of settings.SERVER_EMAIL. The

reviewboard.notifications.tests suite was run. Tests added for new

review emails and review reply emails with enable_smart_spoofing

disabled.