Hook up stripe external account with connect account, expose bank_account info

[paulsullivanjr]

What's in this PR?

This is in progress and not complete. Initial changes to associated the external account with StripeConnectAccount to provide access to the last 4 digits of the account and routing numbers.

References

Fixes #597

[begedin]

@paulsullivanjr In the interest of wrapping this up and launching the feature as soon as possible, I hope you don't mind my taking this PR over. Good work up to this point!

@begedin makes sense. Definitely didn't want to hold you guys up from finishing!

[begedin]

The reason we do this at the service instead of the adapter level is because we want the service to return a useful error response in case the connect account was not found.

Specifically, the service will return {:ok, created_external_account} if everything is ok, and {:error, :not_found} if the associated connect account was not found locally.

Due to this behavior, the event handling system will mark the event as errored.

We could technically push it further into the adapter, but the code would be more complicated.

[begedin]

@joshsmith Should be good to go.

The relationship is stored on the external account record. My reasoning is that we may end up at one point having a

:stripe_connect_account -> has_many -> :stripe_external_account

With that in mind,

:stripe_connect_account -> has_one -> :stripe_external_account

makes more sense at the moment

There is no chance of us having

:stripe_external_account -> has_many -> :stripe_connect_account

so

:stripe_external_account -> has_one -> :stripe_connect_account

makes much less sense.

[joshsmith]

I think we should also add a bank_account_bank_name to this so that we can render that out to the user, as well.