fix success rate math

[danlipert]

Fixes the success rate calculation

[codecov]

Codecov Report

Merging #5167 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5167      +/-   ##
==========================================
- Coverage    30.9%   30.88%   -0.02%     
==========================================
  Files         220      220              
  Lines       17702    17702              
  Branches     2439     2439              
==========================================
- Hits         5470     5468       -2     
- Misses      12006    12007       +1     
- Partials      226      227       +1

Impacted Files	Coverage Δ
app/dashboard/models.py	56.44% <0%> (ø)	⬆️
app/dashboard/utils.py	40.73% <0%> (-0.46%)	⬇️
app/dashboard/views.py	14.41% <0%> (ø)	⬆️

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