Add new distributor property: SKU URL

This is a URL which can be used to look up an item on a distributor's
website, using %s as a placeholder, similar to Firefox keyword
searches.
Adding remaining files for the SKU linking
Only retrieve project once, fixes #343
Merge pull request #342 from johnbeard/linksku

Add new distributor property: SKU URL
Added help trigger on how to enter the SKU placeholder. Relates to #342
Added link trigger image+css. Relates to #342