The purpose of this contract is to mint NFT to proof of attendance for octopus community activities.
owner: The owner of this contract. Some interfaces can only be called by owner.activity: An activity created byactivity creator. An activity has a token metadata and some nfts, all of nfts will use this token metadata.creator: An creator can create lots of activities, and mint nfts for participants of the activity.creator whitelist: Only owner can add or remove creators in whitelist. Only accounts in whitelist are creators.