[#4895] outgoing email logging improvements

[viktorvanwijk]

Closes #4895

Changes

Add custom headers to confirmation and cosign emails

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.57%. Comparing base (3ae753c) to head (9441219).
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4905   +/-   ##
=======================================
  Coverage   96.57%   96.57%           
=======================================
  Files         757      757           
  Lines       25728    25730    +2     
  Branches     3379     3379           
=======================================
+ Hits        24847    24849    +2     
  Misses        616      616           
  Partials      265      265           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sergei-maertens]

I think it's better to have this test in the submissions app instead of appointments, since it's not something exclusive or special to appointments - even for non-appointment submissions we expect these confirmation emails (with custom headers) to be sent.

[sergei-maertens]

This email isn't a cosign confirmation email, but rather a cosign request. To be fair, I think the original code defining the event is wrong and should be renamed to 'cosign-request', since the cosign confirmation is not much different from the "normal" confirmation email (if I remember correctly).

[sergei-maertens]

Nice one!