Cas group allow

[slyngshede]

Allow the user to configure a group whitelist.
Any user not a member of the one or more of the
whitelisted groups will not allowed to
authenticate, if the social_auth.auth_allowed
is enabled.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.82%. Comparing base (d216c96) to head (0362b78).
Report is 4 commits behind head on master.

❗ Current head 0362b78 differs from pull request most recent head c2d74f3. Consider uploading reports for the commit c2d74f3 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #801      +/-   ##
==========================================
+ Coverage   77.77%   77.82%   +0.04%     
==========================================
  Files         347      330      -17     
  Lines       10606    10097     -509     
  Branches      699     1194     +495     
==========================================
- Hits         8249     7858     -391     
+ Misses       2200     2086     -114     
+ Partials      157      153       -4     

Flag	Coverage Δ
unittests	77.82% <100.00%> (+0.04%)	⬆️

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[nijel]

Can you please add tests for this?