Fix task id generation for tutorials

[tnagorra]

Depends on

• Use correct task x and task y for tutorials mapswipe-assets#31

Changes

• use zero index to calculate task id
• add unit test for the calculation

This PR doesn't introduce any:

• temporary files, auto-generated files or secret keys
• n+1 queries
• flake8 issues
• print
• typos
• unwanted comments

This PR contains valid:

• tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.50%. Comparing base (8c053fb) to head (d8a6a5c).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #272      +/-   ##
===========================================
+ Coverage    87.45%   87.50%   +0.05%     
===========================================
  Files          211      213       +2     
  Lines        12837    12877      +40     
  Branches      1089     1085       -4     
===========================================
+ Hits         11226    11268      +42     
+ Misses        1212     1211       -1     
+ Partials       399      398       -1     

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