Modify UI to retrieve solved property in question from user-account-service

[noce2]

In response to the swagger defined here: #7 , the question response will not be aware of any user context. This means an additional mock-backend needs to be set up to represent the user and status of a question they may/may not have attempted.

• A mock backend for the user account service exists (needs to be implemented here: https://github.com/math-dojo/single-page-web-app/blob/master/mock-backends/db.json)
• In the topic view, the UI makes a call to the mock user-account-service to find out whether a question has been attempted/solved by a user (https://github.com/math-dojo/single-page-web-app/blob/master/src/app/components/practice/topic-page/topic-page.component.ts).