Switched to doctrine entities for fixed rates.
Switched to doctrine entities for hourly rate data.
Moved more functions away from the database layer.
Fixed bugs for customer and user login.
Switched to doctrine entities for expense data.
Removed now unnecessary ApiDatabase class.
Removed private db layer from export extensions.
Refactored budget methods from database layer.
Removed database layer from authentication plugins.

The database layer is no longer used in the authentication plugins and
therefore removed.
Switched to doctrine entities for configuration data.
Moved login logic into own class.
Removed unused methods from database layer.
Fixed login issue.