[w3c/push-api] Properly define the pushsubscriptionchange event (#234)

This is a first attempt at #228 and related issues. An [online preview](https://rawgit.com/beverloo/push-api/change-event/index.html) is available.

I added a few inline issues because iteration will be necessary - more things will have to be split out in algorithms. I think that coincidentally improves clarity, so I plan to model the `push` event after `pushsubscriptionchange`, but will do that in a separate PR.

WDYT?
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/push-api/pull/234

-- Commit Summary --

  * Move the ServiceWorkerGlobalScope extensions to their own section
  * Redefine `pushsubscriptionchange`
  * Minor textual updates
  * Fire `subscriptionchange` when changing the encryption keys
  * Fire `pushsubscriptionchange` when refreshing a subscription
  * Maybe fire `pushsubscriptionchange` on revoked permission

-- File Changes --

    M index.html (236)

-- Patch Links --

https://github.com/w3c/push-api/pull/234.patch
https://github.com/w3c/push-api/pull/234.diff

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