Unify CSS theme input between the desktop app and web admin page

Getting a custom CSS theme onto a self-hosted instance currently requires pasting code into a text field with a 32,768 character limit. This is challenging for larger themes, or themes that make extensive use of comments.

The desktop app currently supports uploading or linking to a .css file. Ideally, the web admin page would support this too. This would consolidate the user experience and trade the character limit for a file size limit.

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