Split user management between User and UserRepository

Move static methods for user ID lookup, loading, and resolution
into UserRepository class.

Add an instance of the UserRepository class to the context.

Authentication methods and workspace-related code are updated to call
these methods via the context or pass the database directly.

[ Next steps ]

From dependency injection point of view, this change is "part 1":
it replaces global variables with context-based usage,
but $context still acts a service locator.

From separation of concern standpoint, the plan is to move
all database-related methods into the UserRepository class.

Differential Revision: https://devcentral.nasqueron.org/D3857