Ensure function in case newsletter categories are not enabled

[Tschuppi81]

Newsletter: Adds option to notify admins if a user unsubscribes from the newsletter subscription list

TYPE: Feature
LINK: ogc-2037

Checklist

• I have performed a self-review of my code
• I considered adding a reviewer
• I have updated the PO files
• I made changes/features for org and town6
• I have tested my code thoroughly by hand
  • I have tested sending mails
• I have added tests for my changes/features

[linear]

OGC-2037 Benachrichtigung/Nachverfolgung Abmeldung Newsletter

[codecov]

Codecov Report

Attention: Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.

Project coverage is 88.74%. Comparing base (d2c4c7d) to head (b34c04d).
Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
src/onegov/org/forms/settings.py	80.00%	2 Missing ⚠️
src/onegov/org/views/newsletter.py	80.00%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/onegov/org/forms/newsletter.py	86.59% <100.00%> (+0.22%)	⬆️
src/onegov/org/models/organisation.py	98.00% <100.00%> (+1.51%)	⬆️
src/onegov/org/utils.py	85.64% <100.00%> (+0.06%)	⬆️
src/onegov/org/views/subscription.py	91.17% <100.00%> (+1.52%)	⬆️
src/onegov/org/views/newsletter.py	95.66% <80.00%> (-1.06%)	⬇️
src/onegov/org/forms/settings.py	80.76% <80.00%> (-0.06%)	⬇️

... and 220 files with indirect coverage changes

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