This builds upon the new work in Djblets and the contribution from Brian

LeBlanc to add control over whether a Review Board server always uses

a user's own e-mail address as the From address, never does, or chooses

based on DMARC records.
In previous releases, we had a Django setting for switching between

smart spoofing and always spoofing, but never had any UI support for

this, nor did we have any ability to force the default server address to

be used. Brian's change gave users the ability to use the default server

address through a new checkbox in the E-mail Settings page, but removed

the ability to force spoofing (useful and faster in controlled

environments).
This change switches the checkbox to a drop-down list, giving the

options to automatically choose whether to spoof, always spoof, or never

spoof. It utilizes the new from_spoofing= argument in Djblets's

EmailMessage class, replacing the old enable_smart_spoofing argument

and settings.EMAIL_ENABLE_SMART_SPOOFING setting.
It also renames the "Sender e-mail address" setting to

"Default From address", which is a little more clear, especially alongside

the new option.

Unit tests passed.
Manually tested with all the options, checking the headers of the

resulting e-mails.