This is a preparatory commit for using isort for sorting all of our
imports, merging changes to files where we can easily review the
changes as something we're happy with.
These are also files with relatively little active development, which
means we don't expect much merge conflict risk from these changes.
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.
Tim Abbott
rht
James Rowan