This causes `upgrade-zulip-from-git`, as well as a no-option run of
`tools/build-release-tarball`, to produce a Zulip install running
Python 3, rather than Python 2. In particular this means that the
virtualenv we create, in which all application code runs, is Python 3.
One shebang line, on `zulip-ec2-configure-interfaces`, explicitly
keeps Python 2, and at least one external ops script, `wal-e`, also
still runs on Python 2. See discussion on the respective previous
commits that made those explicit. There may also be some other
third-party scripts we use, outside of this source tree and running
outside our virtualenv, that still run on Python 2.
Both the queue processor and ScheduledJob emails need to sometimes pass a
to_user_id and sometimes pass a to_email, and it's more convenient to just
have one function that they can call that can handle either.
Also removes the now redundant send_email_to_user.
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.
