[Data Model] Make "user" a first-class concept

[mohammedri]

From @hyun20005

In terms of our data model, we conceptually have a User that is associated with every operation, but the implementation of a User is not very well defined. This gets even more blurry when getting into authentication and how the GUI currently deals with the user name.

We should have a discussion fleshing out the User entity within our data model and how we can integrate it into our architecture.