boobank: add degiro module

This module talks to the Degiro stock market broker. The website is fairly complex and has a heavyweight Javascript frontend that does a lot of things itself (such as computing the account balance). That makes the module more complex than others, because retrieving the list of current positions is a multi-step process, and retrieving the current account balance is not feasible trivially. (The reason why is because Degiro has no concept of cash, instead it holds all your cash in "cash funds" which are subcompartments of actual monetary funds that even have an ISIN.)