This will allow for customized senders for emails, e.g. 'Zulip Digest' for
digest emails and 'Zulip Missed Messages' for missed message emails.
Also:
* Converts the sender name to always be "Zulip", if the from_email used to
be settings.NOREPLY_EMAIL_ADDRESS or settings.ZULIP_ADMINISTRATOR.
* Changes the default value of settings.NOREPLY_EMAIL_ADDRESS in the
prod_setting_template to no longer have a display name. The only use of
that display name was in the email pathway.
James Rowan