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.
This adds helpful email notifications for users who just logged into a
Zulip server, as a security protection against accounts being hacked.
Text tweaked by tabbott.
Fixes#2182.
