fix: Delay importing `html.clean_html`

[mbi]

As mentioned in #468, Importing html.clean_html too early prevents registering custom user models containing an HTMLField.

This PR naively moves these imports into the two functions calling clean_html to delay the import and allow registering the models properly.

NOTE: This lacks a proper test, as I couldn't manage to register a custom user model and define it in settings.AUTH_USER_MODEL using django-cms's app_helper mechanism. If you absolutely need a test, can you please provide guidance on how to define a custom user model, that would trigger the error (when 172e1f0348415fa80ce4b622c34587460e162afd is not present)?

[fsbraun]

@mbi Thank you so much for this PR. I'll have to ponder how to create a HTMLField in a test when the apps are not initialized yet. Two things from my side:

1. Could you add a changelog entry?
2. For me the natural decoupling would be to move the CMSPlugin import in utils.py (imported in html.py) into the get_plugins_from_text function. Would that be an alternative?

[codecov[bot]]

Codecov Report

Merging #636 (172e1f0) into master (9ac4c15) will increase coverage by 0.06%. The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #636      +/-   ##
==========================================
+ Coverage   69.86%   69.93%   +0.06%     
==========================================
  Files          16       16              
  Lines         448      449       +1     
  Branches       49       49              
==========================================
+ Hits          313      314       +1     
  Misses        118      118              
  Partials       17       17              

Impacted Files	Coverage Δ
djangocms_text_ckeditor/fields.py	68.88% <71.42%> (+0.70%)	:arrow_up:

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[mbi]

For me the natural decoupling would be to move the CMSPlugin import in utils.py (imported in html.py) into the get_plugins_from_text function. Would that be an alternative?

Yes, I just empirically tested this and it also seems to work. I'll update the PR.

[mbi]

@fsbraun I closed this by accident by force-pushing to remove 172e1f0348415fa80ce4b622c34587460e162afd Here is a new PR with the correct fix and changelog. #637.