close email connections when done

[escattone]

mozilla/sumo#2179 (and part of mozilla/sumo#1993)

Notes

• An instance of the django.core.mail.backends.smtp.EmailBackend class -- which is what django.core.mail.get_connection returns -- is a Python context manager, so it'll automatically take care of opening and closing the connection when used in a with statement.
• This PR ensures that all of SUMO's email connections are closed after they've been used. Every email in SUMO is ultimately sent from either the kitsune.sumo.email_utils.send_messages function or the kitsune.tidings.events.Event.send_emails method.

[akatsoulas]

Small change with big impact, regardless if it fixes the problem or not.

Probably this should go with the ratelimit PR but there's also the send_mass_mail that we could look into which sends multiple emails efficiently with a single connection