Bump Django 2.1.2, remove ColdStartRating

[jilljenn]

• Fix on_delete (now required)
• Remove useless middlewares
• Remove ColdStartRating entirely
• Replace django.core.resolvers with django.urls
• Fix tests (exceptions are now returned by Django)
• Remove scipy from mangaki requirements (but it is still needed in zero)

[codecov]

Codecov Report

Merging #636 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #636      +/-   ##
==========================================
- Coverage   82.12%   82.12%   -0.01%     
==========================================
  Files          50       50              
  Lines        2311     2310       -1     
==========================================
- Hits         1898     1897       -1     
  Misses        413      413

Impacted Files	Coverage Δ
mangaki/mangaki/utils/archive_export.py	36.84% <ø> (ø)	⬆️
mangaki/mangaki/utils/recommendations.py	98.14% <ø> (-0.04%)	⬇️
mangaki/mangaki/settings.py	72.54% <ø> (ø)	⬆️
mangaki/mangaki/wrappers/anilist.py	87% <100%> (ø)	⬆️
mangaki/mangaki/utils/work_merge.py	91.83% <100%> (ø)	⬆️

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