Reduce SQL queries for ordering watchlist

[jilljenn]

• The number of queries is drastically reduced
• Repair tests (some of them make no sense to me)
• We REMOVE the natsort dependency! ← @RaitoBezarius you should be happy
• Actually it is not that clever to put available_works in the Django queryset, should fix before review

[codecov]

Codecov Report

Merging #797 (f71666b) into master (6f3471b) will increase coverage by 0.18%.
The diff coverage is 76.78%.

@@            Coverage Diff             @@
##           master     #797      +/-   ##
==========================================
+ Coverage   79.23%   79.42%   +0.18%     
==========================================
  Files          66       66              
  Lines        4561     4549      -12     
==========================================
- Hits         3614     3613       -1     
+ Misses        947      936      -11     

Impacted Files	Coverage Δ
mangaki/mangaki/utils/fit_algo.py	85.18% <ø> (ø)
mangaki/mangaki/utils/recommendations.py	94.38% <69.23%> (+7.28%)	⬆️
mangaki/mangaki/utils/profile.py	67.18% <76.31%> (+4.50%)	⬆️
mangaki/mangaki/utils/ratings.py	92.85% <100.00%> (+0.26%)	⬆️
mangaki/mangaki/views.py	74.00% <100.00%> (-0.29%)	⬇️

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