Make users' "Preferred streaming service" an ordered list of fallbacks

[szc126]

Currently, a user is only able to select one service under My settings > Interface > Preferred streaming service.

If

• I select "YouTube" (because I like Google's speed)
• but also prefer only audio (piapro, SoundCloud, etc.)
• and don't like NND (for various reasons)

and

• An entry has only
• 1. NND and
• 2. bilibili and
• 2. piapro

VocaDB will choose NND for me. It would be nice if a user could create a list of fallbacks instead.

[riipah]

I'll consider it, but it won't be top priority.

[github-actions]

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