Added basic repository support for identity table
Created by: sbryzak
This PR introduces a db schema and support for some database-related features such as schema migration. It also implements a basic repository object for managing and persisting identity objects. We will almost definitely need to introduce additional properties to the Identity table however for now the requirements aren't absolutely clear so this PR will limit it to just the Username property. The important part of this PR is the testing and other infrastructure which is required when adding additional repository objects.