Statistics endpoints for schoolsyst itself

[ewen-lbh]

Will close #17.

To do:

• Add tests

[codecov]

Codecov Report

Merging #24 into master will decrease coverage by 1.15%.
The diff coverage is 55.81%.

@@            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     

Impacted Files	Coverage Δ
schoolsyst_api/global_stats/routes.py	44.11% <44.11%> (ø)
schoolsyst_api/global_stats/models.py	100.00% <100.00%> (ø)
schoolsyst_api/main.py	96.15% <100.00%> (+0.32%)	⬆️

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