feat!(export): export categorized keywords for apps

[Fenrikur]

BREAKING CHANGE: export for apps returns JSON containing categorized keywords instead of fixed set of categories

Former export fields Cat. Prints, Cat. Artwork, Cat. Fursuit, Cat. Commissions and Cat. Misc replaced by field Keywords containing a JSON object with categories as keys and an array of applicable keywords for each category as value. UTF-8 characters will be escaped.

Example:

{
	"Home and Bedding D\u00e9cor": [
		"Coasters",
		"Towels",
		"Furnishings (Other)"
	],
	"Music and Audio": [
		"Music and Audio (Other)"
	]
}