On install/enable, redirect user to new interface.

Created by: twomice

The current user experience for in-app installation of this extension is like this:

1. Navigate to Administer > System Settings > Extensions for the legacy Extensions page.
2. Find this extension in the list, and click Install; on the confirmation screen, click Install.
3. Observe that you're redirected back to the legacy Extensions page.
4. Wonder what's changed. I installed the new extensions UI, but this still looks just like the legacy Extensions page (because it is).

I'm not sure if we can intercept CiviCRM's navigation early enough and intelligently enough to actually redirect the user to the new Extensions UI page. Perhaps it's enough to just put a message on the legacy Extensions page saying, "New Extension UI is installed and available. Click here to use it."