integrate `model` into `0pdd`

[h1alexbel]

Lets integrate model module into 0pdd directly.
For now it only works as external, self-module.

We should wire it with the 0pdd service (with an ability to turn it off, like it described here; see model attribute)

if its enabled, it will run the ranking on puzzles, otherwise it will do nothing with ranking like plain old pdd

[0pdd]

@h1alexbel 3 puzzles #562, #563, #564 are still not solved.