Migrate "defaultRoute" advanced setting

[flash1293]

#63443 changed the default setting of defaultRoute to /app/home.

For users who already configured a defaultRoute starting with app/kibana, this will cause a double page load from now on in most cases (the only exceptions are default routes pointing to management).

This setting should be migrated. As the new paths are not known during migration (because this information is set in the setup methods of the respective plugins), it should be set to app/url_migrate. This will do the redirect of the legacy path to the right app client side without a page load.

If the URL starts with app/kibana#/management, it shouldn't be migrated yet.

We should hold off this task until near the feature freeze of 7.9 - if management is migrated till then, it won't be necessary to put in a special rule for it.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)