Add Drush command and UI button to clear non-volatile storage
This adds a clearStorage() method to the EnabledSourceHandler service. It then invokes that service from a Drush command and from the UI.
I took the form modeling from the /admin/config/development/performance page, where it smashes two separate forms together using a Controller. I did not, however, wrap all the existing form in a details in the way that page does. I figure it may be out of scope for this issue, and I'm not 100% convinced it's a better UI. But I would do it if told. :)
Closes #3469860