Notify 'domain owner' feature

[robinbzk]

User story:
As a domain portfolio manager who uses the Internet.nl dashboard,
I want the dashboard to send email notifications to individual domain owners about non-compliance and negative changes in compliance for REQUIRED subtests,
so that a domain owner is automatically triggered to take action to fix noncompliance.

Context:
Currently a domain portfolio manager has to individually notify many domain owners in case of noncompliance. Doing this by hand (individual emails) is time consuming. Most government domain portfolio managers do not have the time to follow up on reports on a structural basis.
By automating the notification process, the workload is reduced, and noncompliant domain owners are triggered more often to fix compliance issues, which should lead to higher adoption of internet standards.

Considerations:

• This requires a domain portfolio manager to link email addresses to domains. The email addresses have to be encrypted to mitigate data leaks.
• In the notification, it should be made clear on who's behalf the notification was sent (the dashboard user first and last name and email address should be stated).
• The notifications could be triggered by having the dashboard user manually trigger the notifications, or they could be linked to recurring scans.
• An opt-out is probably a good idea.
• E-mail adresses should be able to be uploaded in bulk, probably similar to how tags are handled
• Some domains may have multiple associated domain owners.
• Perhaps the email notification should have a "Reply-To" to the Dashboard user emailaddress.

[stitch]

Also see #252