feat: Add abstract base model AbstractFrontendUIItem

[fsbraun]

As discussed in #194 the FrontendUIItem model class does not allow adding model fields, since this would imply subclassing CMSPluginModel twice which is not possible.

To remedy the situation, this PR introduces an abstract base model AbstractFrontendUIItem which by default has one concrete model FrontendUIItem.

If users need to create frontend models with additional model fields, they can do so using AbstractFrontendUIItem.

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 86.96%. Comparing base (f11feee) to head (0e7fd74).
Report is 5 commits behind head on master.

Files	Patch %	Lines
djangocms_frontend/contrib/link/models.py	0.00%	1 Missing and 1 partial ⚠️
djangocms_frontend/contrib/link/forms.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
+ Coverage   86.93%   86.96%   +0.02%     
==========================================
  Files         120      120              
  Lines        3107     3114       +7     
  Branches      330      330              
==========================================
+ Hits         2701     2708       +7     
  Misses        304      304              
  Partials      102      102              

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