feat: Universal support for django CMS v3 and v4

[fsbraun]

If PR merges django cms v4 compatibility into the current develop branch. It's objective is to unite the two current develop branches (develop and support/4.0.x) into one branch so that the next release of djangocms-text-ckeditor will support versions 3 and 4 of django CMS universally.

This PR essentially adds the changes of the support/4.0.x branch to the develop branch and adds appropriate case handling for django CMS v3.x and v4.

• The key difference is the way child plugins for a Text plugin are created and deleted.
• The javascript frontend needs to supply the position parameter to create child plugins for v4 (also supplied though not relevant for v3)
• Additionally the improved handling of ghost plugins has been ported forward.
• Fixtures have been added to the tests so they can run both on django CMS v3.x and v4.

Tests pass for Django 2.2, 3.1, 3.2, and 4.0 and django CMS 3.7, 3.7, 3.9., 3.10, 3.11 and 4.0-develop.

utils.py adds an is_cms_v4 property which is True for django CMS v4+ and false for django CMS v3. It also has a property cms_placeholder_add_plugin which contains the unreversed name of the add plugin endpoint (which differs between v3 and v4).

As with the current support/4.0.x branch tests currently run w/o using djangocms-versioning.

[codecov[bot]]

Codecov Report

Merging #631 (5a10d43) into master (c8d5fce) will increase coverage by 6.24%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #631      +/-   ##
==========================================
+ Coverage   69.93%   76.18%   +6.24%     
==========================================
  Files          16       20       +4     
  Lines         449      932     +483     
  Branches       49      124      +75     
==========================================
+ Hits          314      710     +396     
- Misses        118      196      +78     
- Partials       17       26       +9     

Impacted Files	Coverage Δ
djangocms_text_ckeditor/apps.py	100.00% <100.00%> (ø)
djangocms_text_ckeditor/cms_plugins.py	73.89% <100.00%> (ø)
djangocms_text_ckeditor/forms.py	100.00% <100.00%> (ø)
djangocms_text_ckeditor/models.py	89.13% <100.00%> (ø)
djangocms_text_ckeditor/utils.py	89.89% <100.00%> (+0.65%)	:arrow_up:
djangocms_text_ckeditor/widgets.py	81.53% <100.00%> (+0.58%)	:arrow_up:
djangocms_text_ckeditor/__init__.py	100.00% <0.00%> (ø)
... and 1 more

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