On the front/back, we need a way to differentiate mainnet tokens from testnet ones.

[pldespaigne]

This also depend on #72.

• our own smart-contract, we could add a path prefix in our db:
  • before: https://c8-nft-default-rtdb.europe-west1.firebasedatabase.app/titles/0.json
  • after: https://c8-nft-default-rtdb.europe-west1.firebasedatabase.app/<NETWORK>/titles/0.json
• OpenSea smart-contract (mainnet or Polygon): This will not use our db at all so we will create a new code logic to fetch the metadata, and we could decide which logic to use (old one on Rinkeby, or new one through their API) depending on the env or MetaMask network.