Count 0 point exercises for completions

[Redande]

When calculating if a student has completed enough exercises for a course, also count completed exercises where the exercise's maximum points is 0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.56%. Comparing base (0a5b7be) to head (fe97dc9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1275      +/-   ##
==========================================
- Coverage   66.45%   63.56%   -2.89%     
==========================================
  Files         144      144              
  Lines        6045     6046       +1     
  Branches     1436     1437       +1     
==========================================
- Hits         4017     3843     -174     
- Misses       1889     2030     +141     
- Partials      139      173      +34     

Flag	Coverage Δ
backend	63.56% <ø> (-2.89%)	⬇️

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