Use the new email-alert-api

DO NOT MERGE:
Dependent on: https://github.com/alphagov/finder-frontend/pull/93

This commit changes email notification to happen via the new http://github.com/alphagov/email-alert-api rather than the existing govuk_delivery api and allows us to use tags instead of urls for sending email alerts. I have also added a refactor in order to DRY up the use of the EmailAlertExporter into one location and each observers registry only now needs to specify the correct formatter.

This work should be deployed after the work to finder-frontend to implement the new email_alert_api there in order for tags to be used for email alerts.