Send Email via the API

- uses the new subject/email from fields present on the templates / service tables
- brings the send email api call into line with the sms one.
- same fields (to/template_id)
- same rules regarding restricted services
- wired in as a task into celery

Requires
- new celery queue
- new env property (NOTIFY_EMAIL_DOMAIN)

scripts/run_celery.sh

@@ -3,4 +3,4 @@
 set -e
 
 source environment.sh
-celery -A run_celery.notify_celery worker --loglevel=INFO --logfile=/var/log/notify/application.log --concurrency=4 -Q sms,sms-code,email-code
+celery -A run_celery.notify_celery worker --loglevel=INFO --logfile=/var/log/notify/application.log --concurrency=4 -Q sms,sms-code,email-code,email