Do not send email alerts for Drug Safety Update

The Drug Safety Update is a monthly newsletter sent by the MHRA with custom
content (the full content of the document body). Therefore we're going to allow
the MHRA to send this out manually rather than alerting the user each time a
new one is published.
Merge pull request #364 from alphagov/prevent-sending-emails-for-dsu

Do not send email alerts for Drug Safety Update