mgr/dashboard: Provide way to "opt in" to enabling the telemetry mgr plugin
Upon a fresh cluster installation, it should be possible for the user to "opt in" and enable the telemetry plugin so we can obtain some metrics and statistics about the user's cluster.
Upon first login, the dashboard should display a prompt that informs the user about the purpose of the metrics collection and should ask the user for confirmation to enable it. Optionally, it would be helpful to prompt the user for an email address (optional) and display a sample report of the data that is sent to telemetry.ceph.com.
Even if the user initially rejects the activation, the dashboard should still provide an easy way to enable this plugin later.