Stop using navigation properties in UserStore

- Will follow up in a different PR with fixup/loading of navigation properties (they are not populated correctly today)
- Also adds some tests that load from the database (confirming that navigation properties aren't loaded today)

cc @suhasj @divega 
If you are not planning to use navigation properties anymore (which so far has been your choice :smile:), you should get rid of this comment and write three additional queries to explicitly load Claims, Logins and Roles related to the user. However you are starting with an Expression<Func<TUser, bool>> filter, and EF does not currently support navigation properties, so you will have to do some refactoring around this code or rewrite the expressions :smile:

On the other hand, @anpete believes this query (e.g. the one you get if you uncomment the `Includes()`) is already supported by EF7, but there is a bug somewhere else in which we are not configuring the navigation properties correctly. 

Given that I would recommend embracing navigation properties for the EF implementation of the store, at least for this part.