Check the code quality score for Kotlin and improve if necessary

[swsms]

It seems that now the code quality scoring for Kotlin works slightly worse than for Python or Java.
We need to identify such situations using tests and adjust the scoring.

[nbirillo]

@swsms We changed grades thresholds according to the empirical analysis of 250k students solutions for each language (in total, 1M solutions). See a part of PR#117