Add html body templates for email alerts

This adds templating to allow specialist-publisher to send better looking
emails via the email-alert-api. These templates can be overwritten on a
per format basis.
Overwrite templating for Drug device alert emails

In order to add the MHRA email address into the email body we
overwrite the orginal email templates providing a more specific
template.
Merge pull request #367 from alphagov/feature/email-headers-and-footer

Email headers and footer