Add StripeConnectAccountController update for setting external account

[joshsmith]

Problem

We want to add an update on StripeConnectAccountController to allow for setting external_account to a bank account token string.

We likely want to listen for the bank account being created inside a webhook rather than trying to deal with it in one go, to reduce complexity. Can be a separate task.

[joshsmith]

@begedin are we expanding this into more of an update to handle the other fields from Ember and render out other data that Ember is expecting?