Deprecate the user tag

[tarakby]

Context

The user tag has been deprecated from the Flow protocol long time ago (onflow/flow-go#2171). It was used by FCL verification contract (https://github.com/onflow/fcl-contracts/blob/0dd0c7d/cadence/contracts/FCLCrypto.cdc#L140) during a transition period to give time for wallets and developers to update and it should be removed afterwards (in particular for the account proof verification).

It would be great to:

• deprecate the user constant "FLOW-V0.0-user" from the contract
• make the contract accept a parameter tag to verify user signatures (FCL-JS would set the tag it needs)
• keep only "FCL-ACCOUNT-PROOF-V0.0" for account proof verifications
• update FCL-JS accordingly. FCL-JS can either:
  • force everyone to use "FCL-USER-V0.0" for user signatures
  • allow dapps to set their own tags (preferred to separate domains)

I let the FCL-JS experts decide on what's the best user-friendly change to implement.