feat: Update dark mode switch to be compatible with Django 4.2 admin dark mode

[fsbraun]

Since version 4.2 Django's admin has its own dark mode toggle. While functionally similar, it uses a different naming convention. This PR adopts Django's naming convention

• The color scheme is defined by the data-theme attribute of the html tag (before: data-color-scheme)
• The toggle toggles between three states: light, dark, auto (before: two states - light and dark)

This implementation tries to be backwards compatible and take Django 4.2's theme setting - if not present it falls back to django CMS' color-scheme setting.

If Django 4.2 theme setting is present but is undesired, the user can remove it by typing localStorage.removeItem("theme").

This PR integrates with https://github.com/django-cms/django-cms/pull/7549.

[codecov[bot]]

Codecov Report

Merging #491 (2b8293c) into master (7dea618) will not change coverage. The diff coverage is n/a.

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  Hits            33        33