Inventory should restrict buying NFT without deposit into a safe

[porkbrain]

          I think we could also add an endpoint `buy_nft_into_safe` (or something like that) that takes a reference to a safe that already exists.

Originally posted by @Suficio in #270 (comment)

@bausano:

This sort of leads into a question: how can a collection owner say they want the NFTs to be deposited into safe only. Because that is the royalty guarantee. Would they have to whitelist and build their own endpoint?