Change kudos_token model to match new contract

[jasonrhaas]

Now that we are supporting metadata in the Kudos.sol contract, it might make sense to make the kudos_token model structure to the contract.

This means taking most of the columns that are in the kudos_token table and making them a jsonb field. So accessing them via a Django query would change to something like filter(metadata__name='pythonista').

Other tables in the Gitcoin app already use the jsonb field, so this isn't too unconventional.

The new metadata json field would have a format similar to:

https://github.com/mbeacom/gitcoin-erc721/blob/add-metadata/metadata.json

[jasonrhaas]

Also, tags will be stored as an array in the database, which is much easier to work with.