Splitted models module into sub-modules

[phsym]

The models module has been splitted into many submodules as per #22. Submodules are

• auth
• link
• site
• user

All of them have been re-exported from the parent module models, hiding the internal sub-modules layout so the rest of the code doesn't need to be changed

[qcam]

@phsym Could you please resolve the conflict then we can merge this?

[phsym]

I'll do that

[phsym]

@qcam Done

[qcam]

@phsym thank you. ❤️