Pro 1418 volunteer tickets

[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: Volunteer Ticket

Volunteer registrations now create a ticket. The states can be changed in the ticket and mails are sent for registration and finalization.

TYPE: Feature
LINK: PRO-1418

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 styling changes/features on different browsers
  • I have tested javascript changes/features on different browsers
  • I have tested database upgrades
  • I have tested sending mails
• I have added tests for my changes/features
  • I considered using browser tests für javascript changes/features

[codecov]

Codecov Report

❌ Patch coverage is 80.98160% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.54%. Comparing base (4997ade) to head (6958033).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/onegov/org/models/ticket.py	78.48%	17 Missing ⚠️
src/onegov/feriennet/views/volunteer.py	81.57%	14 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/onegov/activity/collections/volunteer.py	82.60% <ø> (+4.34%)	⬆️
src/onegov/activity/models/volunteer.py	100.00% <ø> (ø)
src/onegov/feriennet/views/settings.py	76.47% <100.00%> (+1.24%)	⬆️
src/onegov/feriennet/views/volunteer.py	72.91% <81.57%> (+29.25%)	⬆️
src/onegov/org/models/ticket.py	88.25% <78.48%> (-0.90%)	⬇️

... and 4 files with indirect coverage changes

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