Create iTip messages for attendee on first upload

[KristofferFM]

https://github.com/sabre-io/vobject/issues/719

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 98.78%. Comparing base (96c3152) to head (a5eca62). :warning: Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #720   +/-   ##
=========================================
  Coverage     98.78%   98.78%           
- Complexity     1877     1880    +3     
=========================================
  Files            71       71           
  Lines          5274     5279    +5     
=========================================
+ Hits           5210     5215    +5     
  Misses           64       64           

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