Optimize quiz page size: do not load all tests at once

[koorchik]

The problem. If you go to https://wall.itsquiz.com/activations/582b4e032e8ee9741a6136cc/test-how-creative-are-you

You'll see this

in developers console.

The main issue is that we load all tests by the same author. We should load only first 8 and add rest of the quizzes while a user scrolls down (like on main page).