Fix lesson pass check

[JasirZaeem]

fixes #966

Fixes incorrect passed lesson computation by counting only 1 submission per challengeId in a lesson by using a distinct clause.

The submissions.count query has been replaced by a findMany because count does not support the distinct clause. Only challengeId for each submission is fetched to fetch minimal extra data from the the db

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[codecov]

Codecov Report

Merging #967 (aeac976) into master (ea4e07e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #967   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          122       122           
  Lines         2020      2020           
  Branches       492       492           
=========================================
  Hits          2020      2020           

Impacted Files	Coverage Δ
helpers/updateSubmission.ts	100.00% <100.00%> (ø)