Update previews, add relatedAddons, remove popup script/html

[cobaltt7]

I just made these use strings so they can be extended in the future without updating the schema.
Should fix SA/SA CI.

[Samq64]

I think presetPreview should keep full validation since it doesn't just take an addon ID like addonPreview does.

Now that I think about it might make more sense for addonPreview to just be a boolean, but that's another issue.

[Samq64]

@cobaltt7 Do you mind of I push to this branch?

[cobaltt7]

go ahead

[Samq64]

The 2nd commit is for ScratchAddons/ScratchAddons#6104 and ScratchAddons/ScratchAddons#7941.