feat: Add django 5.0 support

[fsbraun]

Description

This PR adds Django 5.0 support to django-filer:

• Replace use of Form's undocumented _html_output (removed in Django 5.0) by officially documented AdminForm API (regression test added)
• Replace removed django.core.file.storages.get_storage_class
• Add tests for Django 5.0
• Remove filter_horizontal from ClipboardAdmin (which is inaccessible to the user)
• Adjust admin reverse for swapped image model in tests

Related resources

• https://www.djangoproject.com/weblog/2023/sep/18/django-50-alpha-1-released/
• #...

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[codecov]

Codecov Report

Patch coverage: 92.30% and project coverage change: +0.74% 🎉

Comparison is base (dbeda3a) 75.22% compared to head (5142fbc) 75.96%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1424      +/-   ##
==========================================
+ Coverage   75.22%   75.96%   +0.74%     
==========================================
  Files          75       75              
  Lines        3455     3454       -1     
  Branches      555      554       -1     
==========================================
+ Hits         2599     2624      +25     
+ Misses        691      669      -22     
+ Partials      165      161       -4     

Files Changed	Coverage Δ
filer/admin/clipboardadmin.py	94.66% <ø> (-0.08%)	⬇️
filer/admin/folderadmin.py	78.69% <ø> (+3.04%)	⬆️
filer/admin/forms.py	80.48% <91.66%> (+12.19%)	⬆️
filer/settings.py	81.55% <100.00%> (ø)

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[vinitkumar]

Looks good to me. Let's get this merged and released 🚢 🚀