Using <spark-dialog>

@devoncarew @financeCoding

...and this CL is actually using the new <spark-dialog> and <spark-dialog-button> in the app. It depends and includes everything in #2176, so please review just `ide/app/*`.

I'm splitting the work I've done on the dialogs and daisy-chaining the CLs to make reviewing easier. The next one will deal with dialog styling, and so on.