Generate routes and nav links based on manifest

Created by: gracewashere

Extracting application-specific configuration into a DashboardManifest class. The class will be customized by developers on a per-application basis, and will determine what routes and navigation links get generated. https://administrate-docs.herokuapp.com/getting_started

• Add a dashboard manifest class
• dynamically generate resource routes
• dynamically generate a root route
• dynamically generate sidebar links