Implement longer-term GoCardless API credentials

[diegotsutsumi]

User story

As a personal_finance's user, I don't want to have to input my bank credentials into the browser every time I run fetch_transactions to sync my bank data with my local data.

Tech notes

Implementing an authorization endpoint in the cloud is a solution that solves two problems:

1. The user long-term credentials, by storing it in a cloud database.
2. The link can be access through the mobile browser, making it easy to integrate with bank apps for authentication.

Acceptance criteria

• An API with an authorize endpoint is implemented asking user bank authorization and persisting gocardless credentials.
• The endpoint is deployed to AWS and publicly available through the browser.
• The endpoint implements HTTPS and has a password to access it (until a proper user model is designed).
• fetch_transaction stops asking the user for bank credentials, and starts using persisted credentials gotten by authorize command.

[leonardohscastilho]

The long-term credentials is already implemented, and the cloud storage implementation will be done in next steps https://github.com/in-fin-neat/in-fin-neat-core/issues/40.