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.
Server settings should just be added to the context in build_email, so that
the individual email pathways (and later, the email testing framework)
doesn't have to worry about it.
Previously the rendering code in test_emails.py did not match the rendering
code in send_email.py. This commit removes the duplication to reduce the
chance they drift in the future.
This commit also changes test_emails.html to ensure that we always display
both the HTML and text versions of an email.
This commit replaces all uses of django.core.mail.send_mail with send_email,
other than in the password reset flow, since that code looks like it is just
a patch to Django's password reset code.
The send_email function is in a new file, since putting it in
zerver.lib.notifications would create an import loop with confirmation.models.
send_future_email will soon be moved into email.py as well.