[#2199] Replaced django-yubin with django-mailer

[Bartvaderkin]

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.88%. Comparing base (cd7160b) to head (63a378e).

❗ Current head 63a378e differs from pull request most recent head 3246ff9. Consider uploading reports for the commit 3246ff9 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1090      +/-   ##
===========================================
- Coverage    95.00%   94.88%   -0.12%     
===========================================
  Files          896      894       -2     
  Lines        31449    31421      -28     
===========================================
- Hits         29879    29815      -64     
- Misses        1570     1606      +36     

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[Bartvaderkin]

@alextreme This should be good for testing.

I'm currently updating the 'send_failed_mail_digest' cronjob I disabled to get this green quickly.

I added the fixed 'send_failed_mail_digest' as a PR on this PR Merged it to here after review

[alextreme]

Only one way to check this...