Add BankGateway

[paroga]

This is the first step to replace the integrated import functionality with a clean call to an external api endpoint. The full implementation can be found in a branch in my repository.

[wvengen]

It's a little hard for me to review, even with the full implementation, but as far as I can see, it looks fine. I don't full get the unattended user thing, would you mind to explain?

Also, this could really use some documentation. For developers on how the different parts relate to each other. It's probably straightforward for you - but for newcomers some higher-level description would be good. Perhaps as yardoc class documentation, or else something in doc/.
And ultimately also for users, in what functionality this brings and how to set this up, and e.g. which countries and banks support this kind of interaction. But that's for later.

[paroga]

I don't full get the unattended user thing, would you mind to explain?

if you have a cron-job for importing transactions (aka unattended import) the gateway still needs a user on whose behalf the import happens and who is responsible for configuring the access. usually the credentials to log into you bank account are bound to one specific person and not to an organization

i 100% agree that we need documentation how the bank gateway api works, but i don't see a need to document the data model of this PR. a sane developer can look into app/models/bank_gateway.rb and read has_many :bank_accounts to see how it relates in the remaining model.

[wvengen]

Ah, that makes sense. Thanks for explaining about the unattended user.

Ok, if we have user documentation, then the data model will be clear enough.

[wvengen]

Curious to see what will follow :)