By default, set the fullname from first+last and vice-versa

[dehnert]

Fixes #678.

Proposed changes

As described in #678 , when the SAML backend supplies a fullname, it's desirable for my Django user to have first_name and last_name set. This changes PSA so if fullname is set (in, eg, a SAML response) but not first_name/last_name, by default it generates a first_name and last_name automatically. Similarly, it does the reverse transformation when fullname is missing. IMO this is a net improvement, but obviously there's questions like "should this be enabled by default", "should it be in the default pipeline", etc. -- I'm happy to tweak this to make it fit the team's design goals better. Assuming the concept makes sense, I can write docs.

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• [ ] Release (new release request)
• [ ] Bugfix (non-breaking change which fixes an issue)
• [X] 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)
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[codecov[bot]]

Codecov Report

Merging #683 (3cfc739) into master (f27461d) will decrease coverage by 0.06%. The diff coverage is 10.00%.

@@            Coverage Diff             @@
##           master     #683      +/-   ##
==========================================
- Coverage   77.01%   76.94%   -0.07%     
==========================================
  Files         319      319              
  Lines        9677     9687      +10     
  Branches     1036     1040       +4     
==========================================
+ Hits         7453     7454       +1     
- Misses       2072     2081       +9     
  Partials      152      152              

Flag	Coverage Δ
unittests	76.94% <10.00%> (-0.07%)	:arrow_down:

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Impacted Files	Coverage Δ
social_core/pipeline/__init__.py	100.00% <ø> (ø)
social_core/pipeline/social_auth.py	78.18% <10.00%> (-15.16%)	:arrow_down:

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[dehnert]

Any thoughts on this general feature?