Create iTip messages for attendee on first upload

[KristofferFM]

#719

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.77%. Comparing base (2ce84f7) to head (e5d557d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #720   +/-   ##
=========================================
  Coverage     98.77%   98.77%           
  Complexity     1884     1884           
=========================================
  Files            71       71           
  Lines          5289     5289           
=========================================
  Hits           5224     5224           
  Misses           65       65           

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[phil-davis]

@KristofferFM could you rebase and resolve conflicts please.
There was another PR that also touched code in the same area.

[KristofferFM]

@phil-davis sure, I expect to have something ready within at most a few days.

[KristofferFM]

@phil-davis The merge conflict was due to a functionally equivalent fix of the issue by SebastianKrupinski. So that is all good :-) I have left the test in the PR, up to you if it should be pulled or not, I don't know if there are other tests covering the same area.