Feature: enforce user creation security

[syphax-bouazzouni]

This PR enforces the user creation/deletion security.

Now only an admin user can create an admin user or update a user to an admin, and only admin users can delete another user.

More detail can be found in the added tests.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2c3cba8) 71.85% compared to head (6e68eaf) 71.88%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #131      +/-   ##
===========================================
+ Coverage    71.85%   71.88%   +0.02%     
===========================================
  Files           52       52              
  Lines         2878     2881       +3     
===========================================
+ Hits          2068     2071       +3     
  Misses         810      810              

Flag	Coverage Δ
unittests	71.88% <100.00%> (+0.02%)	⬆️

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[alexskr]

@syphax-bouazzouni, when making pull requests against develop branch, ideally we should keep gem branch specifier set at develop in the Gemfile
in this case, is there a way to remove [0402dc4] (0402dc4) commit?

[syphax-bouazzouni]

@syphax-bouazzouni, when making pull requests against develop branch, ideally we should keep gem branch specifier set at develop in the Gemfile in this case, is there a way to remove [0402dc4] (0402dc4) commit?

Done,
Sorry for that, it was because I started my branch from master not develop.