Add machine translation banner on pages with other languages

A new gadget (called WishlistTranslation) that adds a banner at the top of wishlist pages when there is any text on a page in a language other than the current interface language. Each text node in that language is then sent for translation (this may not be the way to go, as sentence fragments are then sent without context; but the alternative seems to be to break HTML structures; more development is needed).

This doesn't aim to finish the feature, but to just get the banner and fetching and error aspects working.

Bug: T371098