Pro 1448 new recipient group

[BreathingFlesh]

Please fill in the commit message below and work through the checklist. You can delete parts that are not needed, e.g. the optional description, the link to a ticket or irrelevant options of the checklist.

Commit message

Feriennet: New recipient group

Add two new recipient groups:

• Organisers without occasions
• Users with attendees that have no wishes or bookings

TYPE: Feature
LINK: PRO-1448

Checklist

• I have performed a self-review of my code
• I considered adding a reviewer
• I have updated the PO files
• I have tested my code thoroughly by hand
  • I have tested sending mails
• I have added tests for my changes/features

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.33%. Comparing base (666d26e) to head (dd63036).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/onegov/feriennet/app.py	97.72% <100.00%> (+0.08%)	⬆️
...rc/onegov/feriennet/forms/notification_template.py	93.08% <100.00%> (+4.42%)	⬆️

... and 2 files with indirect coverage changes

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