[FEATURE] Add external account model
[FEATURE] Change accountId to externalAccountId in external transactions migration
[FEATURE] Add migration logic for creating external_accounts table.
[FEATURE] Change accountId to externalAccountId in external transactions model
[FEATURE] Work on making gatewayAccount object relation queries execute properly.
[FEATURE] Add external accounts and transactions routes and handler actions
[FEATURE] Add function to handle new ripple notifications