(un)solved problems

[aprooks]

• added solved property to stub API
• loading ProblemsCtrl via resolve
• extracted PronlemsList to external folder

need a feedback for these ideas

[dinoboff]

So GET /content-delivery/problems array items should return a solved property. That's fine for a small set of problems.

To handle larger set, we will need to add to GET /content-delivery/problems an optional query parameter to filter the problem on the server side.

We will also need to page the problems/videos query requests and return a limited number of problems/video per request. We need to wrap the array of videos/problems and include a cursor property.