Dep(#317): Refactor mintable nft logic and rename

[elise-ng]

Ref oursky/likecoin-chain#244 Depends on oursky/likecoin-chain#317

• Rename mintable nft to blind box content
• Refactor blind box state fields in class data
• Refactor msg server files

[hochiw]

@chihimng please help to resolve conflict 🙏