Add collapsible inlines support.

[fabiocaccamo]

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name: Pull request
about: Submit a pull request for this project
assignees: fabiocaccamo

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Describe your changes
This PR brings the following improvements:

• it's possible to make inlines collapsible (via theme admin option)
• it's possible to make collapsible inlines collapsed by default (via theme admin option)
• it's possible to set both options (collapsible/collapsed) separately for stacked and tabular inlines

Related issue
Fix #263

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added tests for the proposed changes.
• I have run the tests and there are not errors.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05 🎉

Comparison is base (a8897a9) 97.29% compared to head (f5d4e6d) 97.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #282      +/-   ##
==========================================
+ Coverage   97.29%   97.34%   +0.05%     
==========================================
  Files          37       38       +1     
  Lines         406      414       +8     
==========================================
+ Hits          395      403       +8     
  Misses         11       11              

Flag	Coverage Δ
unittests	97.34% <100.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
admin_interface/admin.py	100.00% <ø> (ø)
...0030_theme_collapsible_stacked_inlines_and_more.py	100.00% <100.00%> (ø)
admin_interface/models.py	100.00% <100.00%> (ø)

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