Changes whitelist handling to cope with jupyterhub.

[stefanoborini]

Addresses the following issues:

• adds config=True to allow the option to be set at config time.
• returns a copy of the set, because jupyterhub changes it.
• added mock setter to pacify jupyterhub.

[codecov-io]

Codecov Report

Merging #359 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #359      +/-   ##
==========================================
+ Coverage   95.23%   95.25%   +0.01%     
==========================================
  Files          92       92              
  Lines        3885     3895      +10     
  Branches      253      253              
==========================================
+ Hits         3700     3710      +10     
  Misses        127      127              
  Partials       58       58

Impacted Files	Coverage Δ
.../jupyterhub/auth/github_whitelist_authenticator.py	100% <100%> (ø)	✅
.../auth/tests/test_github_whitelist_authenticator.py	100% <100%> (ø)	✅

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