[FEATURE] Add gateway account model methods for ripple address, txns.
[FEATURE] Add endpoint for getting externalTransactions of an account.
[CHORE] Remove unused model, controllers and migration
[FEATURE] Add migration files for external transactions and external accounts.
[FEATURE] Rename gateway_transactions controller to external_transactions
[CHORE] Remove references to BankTx
[FEATURE] Add model methods for querying externalAccounts and externalTransactions of a GatewayAccount.
Merge branch 'model-associations'