feat: Add django 5.0 support

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

Checklist

[x] I have opened this pull request against master
[x] I have added or modified the tests when changing logic
[x] I have followed the conventional commits guidelines to add meaningful information into the changelog
[x] 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 Report

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

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

```diff @@ 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](https://app.codecov.io/gh/django-cms/django-filer/pull/1424?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=django-cms) | Coverage Δ | | |---|---|---| | [filer/admin/clipboardadmin.py](https://app.codecov.io/gh/django-cms/django-filer/pull/1424?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=django-cms#diff-ZmlsZXIvYWRtaW4vY2xpcGJvYXJkYWRtaW4ucHk=) | `94.66% <ø> (-0.08%)` | :arrow_down: | | [filer/admin/folderadmin.py](https://app.codecov.io/gh/django-cms/django-filer/pull/1424?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=django-cms#diff-ZmlsZXIvYWRtaW4vZm9sZGVyYWRtaW4ucHk=) | `78.69% <ø> (+3.04%)` | :arrow_up: | | [filer/admin/forms.py](https://app.codecov.io/gh/django-cms/django-filer/pull/1424?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=django-cms#diff-ZmlsZXIvYWRtaW4vZm9ybXMucHk=) | `80.48% <91.66%> (+12.19%)` | :arrow_up: | | [filer/settings.py](https://app.codecov.io/gh/django-cms/django-filer/pull/1424?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=django-cms#diff-ZmlsZXIvc2V0dGluZ3MucHk=) | `81.55% <100.00%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

django-cms / django-filer