Split concerns between Users and UserRepository

Summary:
Promote User in Waystone\Engines\Users namespace and move anything
related to database operations for users table into UserRepository.

Methods related to other objects still need to be sorted, perhaps
to new repositories.

Test Plan:
Change at risk: site still able to login, logout, load session,
but we don't have Selenium or unit tests for remote identity provider.

Differential Revision: https://devcentral.nasqueron.org/D3858