[server] load leaderboards from mongo based on topic and diffictuly

[yishain11]

I want to add a competition element to our project, so we need to add support for that in mongo.

DOD:

• [x] add the collections nessesary for storing user id, points - per topic. (i.e. python: user1: X points, user2: Y points...)
• [x] at server startup, make sure that this collection exists. if not - created it. make sure not to delete anything by accident
• [x] tests, logs, err handeling etc