`EmailChannel.send_now` fallback templates

[kevinrenskers]

EmailChannel.send_now uses the following templates to send an email:

subject_template = f"notifications/email/realtime/{notification.notification_type}_subject.txt"
html_template = f"notifications/email/realtime/{notification.notification_type}.html"
text_template = f"notifications/email/realtime/{notification.notification_type}.txt"

It should fall back to "subject.txt", "message.html" and "message.txt" in case the notification type specific template(s) can't be found. The docs in customizing.md will need to be adjusted as well.