fix(settings): Extend ALLOW_TOKEN_PARSERS if custom one is provided

[DmytroLitvinov]

Closes #647

[DmytroLitvinov]

Hi @fsbraun ,
This one should fix it.

Exampe in interpreter that it works:

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (4e5c4bd) 76.33% compared to head (dec185c) 76.33%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #654   +/-   ##
=======================================
  Coverage   76.33%   76.33%           
=======================================
  Files          20       20           
  Lines         917      917           
  Branches      123      123           
=======================================
  Hits          700      700           
  Misses        191      191           
  Partials       26       26           

Files	Coverage Δ
djangocms_text_ckeditor/settings.py	90.00% <ø> (ø)

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