Statistics endpoints for schoolsyst itself

[gwennlbh]

Will close #17.

To do:

• Add tests

[codecov]

Codecov Report

❌ Patch coverage is 55.81395% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.00%. Comparing base (5ac4735) to head (1807f09).
⚠️ Report is 36 commits behind head on master.

Files with missing lines	Patch %	Lines
schoolsyst_api/global_stats/routes.py	44.11%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
- Coverage   80.16%   79.00%   -1.16%     
==========================================
  Files          32       34       +2     
  Lines         862      905      +43     
==========================================
+ Hits          691      715      +24     
- Misses        171      190      +19     

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