Adds VALIDATE_USER setting config, if False then don't check email in User model

[symbolic09]

Need this functionality for the applications that don't have a web-based interface for normal users. if NEWSLETTER_VALIDATE_USER is false in settings.py then it will not check for email in the User model. fixes #369

[codecov[bot]]

Codecov Report

Merging #370 (b9222ff) into master (c350966) will decrease coverage by 0.13%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #370      +/-   ##
==========================================
- Coverage   86.02%   85.88%   -0.14%     
==========================================
  Files          16       16              
  Lines        1302     1311       +9     
  Branches      137      139       +2     
==========================================
+ Hits         1120     1126       +6     
- Misses        135      136       +1     
- Partials       47       49       +2     

Impacted Files	Coverage Δ
newsletter/settings.py	90.00% <71.42%> (-2.46%)	:arrow_down:
newsletter/validators.py	88.88% <80.00%> (-11.12%)	:arrow_down:

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[ekerstein]

Any update on this PR? I would greatly like to use this feature.

My newsletter signup is only public-facing. So I want to require emails to always be inputted and disable the validation to not leak user data per this issue: https://github.com/jazzband/django-newsletter/issues/183

[newearthmartin]

I need this PR too as I also do not use the django user system.