Add configuration to require POST only authentication requsts

[sultaniman]

Proposed changes

This PR introduces a new configuration and authentication behaviour when it is active and will require POST request during authentication

• Configuration key is SOCIAL_AUTH_REQUIRE_POST: bool,
• Error response if request method is not POST.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (PEP8, lint, formatting, renaming, etc)
• Refactoring (no functional changes, no api changes)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build related changes (build process, tests runner, etc)
• Other (please describe):

Related issues and discussions

• Requiring POST method by default #486
• CSRF on login #55

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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.07% 🎉

Comparison is base (4b4b019) 93.99% compared to head (c180450) 94.07%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #493      +/-   ##
==========================================
+ Coverage   93.99%   94.07%   +0.07%     
==========================================
  Files          39       39              
  Lines        1116     1131      +15     
  Branches      129      133       +4     
==========================================
+ Hits         1049     1064      +15     
  Misses         43       43              
  Partials       24       24              

Flag	Coverage Δ
unittests	94.07% <100.00%> (+0.07%)	⬆️

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Files Changed	Coverage Δ
social_django/utils.py	97.56% <100.00%> (+0.78%)	⬆️
social_django/views.py	100.00% <100.00%> (ø)
tests/test_views.py	100.00% <100.00%> (ø)

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[nijel]

Merged, thanks for your contribution!

[sultaniman]

@nijel I will also create a PR to update Django specific configuration documentation.
Thanks for reviews.

[sultaniman]

@nijel I updated docs and here is the PR