feat: add user modules for Yaga, StackB, Pedsovet

[kristoisberg]

Seriously sorry for the previous pull request. I removed the per-module tests and parser fallbacks, which made the modules too complex (they are still more complex than most, since these are not the most simple sites to parse). I also made the two Yaga modules completely self-contained and refactored the modules.

Original description:

This PR adds username modules for several sites I've ran into with usernames exposed in their profile URLs:

• Yaga is an Estonian online marketplace, they operate sites for Estonia, Latvia and South Africa, but the Latvian shares its - database with the Estonian one so I omitted that one.
• StackB is a Russian gaming social network.
• Pedsovet is an ancient Russian site for study materials.

[kaifcodec]

@kristoisberg Thank for the PR, I am tied up with some work so I was unable to test these modules manually, whenever I become free I will test them and let you know.