Split up vorta/models.py

[m3nu]

This splits up vorta.models and puts it in a package vorta.store.

Fixes #937

[m3nu]

Maybe this will also help for the modular refactoring, @bastiencyr .

[bastiencyr]

It can because models mixes different features (migration, models).
But you want to try the module structure so ? Starting with a little experimental implementation ?
Because modules dont change current code in his first implementation or even interact with, we dont need to wait a major version (vorta 0.9 for example).

[m3nu]

Yes, splitting this up will help you with the modular design, I hope. We had too many things in vorta.models.

Starting small is always good. I still have a hard time imagining the changes.