Feat: Basic dark mode compatibility and fix for source edit bug

[fsbraun]

This PR makes djangocms-text-ckeditor integrate with a user's dark mode preference:

• Editor field in html mode retains the system settings colors if no custom css is loaded
• Editor field background in source mode falls back to system settings (of django admin) avoiding to have to edit source code in light gray on white in dark mode
• Max height of editor field in source mode is removed which was introduced in django admin 3.x and lead to a editor field only a few lines high

This PR request does not provide a dark mode skin (ckeditor 4 user interface).

[marksweb]

Thanks @fsbraun!