Add ability to sort statistics results

[danielemery]

Resolves #76

• Can either sort by number of quizzes descending, or average score descending
  • By default it's number of quizzes completed
• Took the opportunity to document and test the whole statistics service

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (625c08e) 8.40% compared to head (9c2a3dd) 20.65%.

Additional details and impacted files

@@            Coverage Diff             @@
##            main      #78       +/-   ##
==========================================
+ Coverage   8.40%   20.65%   +12.24%     
==========================================
  Files         19       19               
  Lines        452      460        +8     
  Branches      65       70        +5     
==========================================
+ Hits          38       95       +57     
+ Misses       400      353       -47     
+ Partials      14       12        -2     

Files	Coverage Δ
src/gql.ts	0.00% <ø> (ø)
src/quiz/quiz.service.ts	48.48% <100.00%> (+20.12%)	⬆️
src/statistics/statistics.service.ts	100.00% <100.00%> (+100.00%)	⬆️
src/user/user.service.ts	0.00% <ø> (ø)
src/statistics/statistics.gql.ts	0.00% <0.00%> (ø)

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