[NFT Claim] Keep token metadata to be claimed at class

[elise-ng]

Ref oursky/likecoin-chain#192

• [x] Refactor NFTInput from MsgMintNFT
• [x] KVStore for claimables, Setup key prefix by class id
• [x] Keeper method to list claimables under a class id, add test case
• [x] Paginated query to list claimables under a class id, add test case
• [x] mutation for create claimable
• [x] mutation for update claimable
• [x] mutation for delete claimable
• [x] read json for nft input at cli, touch up cli ux
• [x] Manually test flow
• [x] emit events
• [x] keeper method to count claimables, make sure we update the count in class data on each mutation
• [x] Plan test cases to follow up

[elise-ng]

On second thought, better store the claimable tokens at another store & make dedicated query (with pagination) and mutation due to data size