remove sandbox from official collection.json

• find out where our top colllection json is used (some international iiif collections overview listing)
• clean the results by not listing sandbox objects
• https://textgridlab.org/1.0/iiif/manifests/collection/sandbox.json
• possibly also split https://dev.textgridlab.org/1.0/iiif/manifests/ in sandbox and non-sandbox area