Allow notifications to be disabled without losing content of notification

Problem/Motivation

Occasionally, I want to import or migrate a large number of entities and not send thousands of emails.

However, if I untick the "Enable notifications" checkbox and save, I lose the content of that notification.

Proposed resolution

Ideally, I want to be able to enable/disable a notification without losing the content to send. I suggest that we add a new item to the schema that holds the notification enabled status, rather than relying on the presence of content in "To".

Remaining tasks

User interface changes

API changes

Data model changes