Add verification_document_id handling to StripeAccountController

[joshsmith]

Problem

StripeAccountController:

• PUT with a verification_document_id
• Pattern match against the verification_document_id in handle_update
• handle_update will call StripeAccountService.add_verification_document()

StripeAccountService.add_verification_document()

with

• Pattern match against the verification_document_id and the Stripe Account information
• Use verification_document_id to fetch the file with Stripe.FileUpload.retrieve (to ensure proper secure data coming from the client)
• Call Stripe.Account.update with verification[document] set to the id from %Stripe.FileUpload{}

do

Inside a transaction:

• Use the returned %Stripe.FileUpload{} to insert a %CodeCorps.FileUpload{}
• Use the returned %Stripe.Account{} to update our %CodeCorps.StripeAccount{}

[joshsmith]

This is blocked by implementing the API in stripity_stripe: beam-community/stripity-stripe#154

[joshsmith]

This is partly blocked by #572 now. Can be resolved by creating a mock for the update in StripeTesting.