Song: Controls to prevent song list data loss from improper merging

[goomie]

The issue with entries 11877 and 3025 demonstrated that improper merges can not only be time-consuming to resolve, but also cause changes to user-made song lists that are practically impossible to revert.

Simple controls that could be added:

• When a Trusted user attempts a merge, block the action or require mod+ approval if both entries belong to song lists (or a certain minimum number of song lists)
• Allow mod+ to edit personal song lists (only for emergency actions like this)

In the long term, automatic song list modifications can be included as part of #587 to rapidly identify which entries were changed in a merge and which belonged to the original entry.

Checklist

• Searched VocaDB/vocadb for duplicate issues.