"requireUsernameProofs" feature flag

Added `requireUsernameProofs` feature flag. When set to `false` username proof is not checked during wallet creation. It's to allow normal (non SDF) stellar-wallet deployments and for stellar-wallet-js-sdk tests to pass (stellar/stellar-wallet-js-sdk#4).
Probably want this to be true in development... We don't want to accidentily break the system because we aren't testing it.