Include controller in Rails generator or use Rails Engine for dashboard

[phillbaker]

The Rails generator currently only creates SQL migrations, we should include a command for generating the controller for the dashboard, tracking image and JS callbacks or move it to a Rails engine or mounted sinatra endpoint.

class VanityController < ApplicationController
    include Vanity::Rails::Dashboard
    include Vanity::Rails::TrackingImage
 end

• [ ] Do as Rails engine, with routes as a mount in config/routes, depends on #132
• [ ] Update README
• [ ] Update documentation for how to use authentication for dashboard while allowing CSRF-protected access to JS callbacks