1159573 - Add IndexedDB migration logic to `PushDB`

Status: RESOLVED DUPLICATE of bug 1160316

(Core :: DOM: Push Subscriptions, defect)

Description

[Lina Butler [:lina]]

`PushDB` uses the same database and object store ("push") as the original Simple Push implementation. This will cause errors for clients with existing push subscriptions. We can rename the original to "simplepush," but that won't update any existing "push" stores. We can also rename the new one, but no sense in keeping both around.

The stores are mostly compatible—for now—and use the same server. We can either rename `manifestURL` to `scope`, or drop existing registrations entirely. Once we migrate to Web Push, we'll want to bump the version again, and drop everything.