Allow per-backend user pipeline settings

[dehnert]

According to https://python-social-auth.readthedocs.io/en/latest/configuration/settings.html, "All settings can be defined per-backend by adding the backend name to the setting name, like SOCIAL_AUTH_TWITTER_LOGIN_URL". This changes user.py to actually use per-backend settings for options like USERNAME_IS_FULL_EMAIL.

Note that get_username is always called with a backend, but user_details isn't, so a different version of `setting' is needed for the two.

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[dehnert]

I'm confused that CI appears to have still not run.

[codecov[bot]]

Codecov Report

Merging #677 (f493c68) into master (f27461d) will not change coverage. The diff coverage is 90.00%.

@@           Coverage Diff           @@
##           master     #677   +/-   ##
=======================================
  Coverage   77.01%   77.01%           
=======================================
  Files         319      319           
  Lines        9677     9677           
  Branches     1036     1036           
=======================================
  Hits         7453     7453           
  Misses       2072     2072           
  Partials      152      152           

Flag	Coverage Δ
unittests	77.01% <90.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
social_core/pipeline/user.py	82.60% <90.00%> (ø)

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[nijel]

Can you please prepare a matching pull request for the documentation? https://github.com/python-social-auth/social-docs/

[dehnert]

I think this change just makes the code match the already-existing documentation -- see https://github.com/python-social-auth/social-docs/blame/master/docs/configuration/settings.rst#L18, "All settings can be defined per-backend". Can you clarify what you're looking for in terms of doc changes?