check collection owner and already branded identity

[bogdan-rosianu]

Two fixes:

1. If a user wanted to brand a (SFT / NFT) collection, the checker would have only checked the owner from the 'tokens' endpoint. Extended the check so it will also check the 'collections' endpoint.
2. If one wants to brand a new identity, there was no check that the identity (for example Staking Provider contract address) wasn't already branded under other names. Added such a check