Make Settings explicitly use a cache

The Settings object used by the background page now uses 1 of 3 caches,
depending on the context it is available in:
* localStorage - in the background page
* a copy of localStorage - in non-background extension pages
  (options.html, popup.html, etc.)
* an empty object - in all other pages (where localStorage doesn't point
  to the extension's localStorage object).

For any extension page which is *not* the background page, a copy of
localStorage is used instead of true localStorage:
* Once localStorage is updated by one background page, the others can
  only see the updated copy.
  - Pages with an updated cache can't tell which changes are new, and so
    don't know which postUpdateHooks to run.
* By copying localStorage's contents into a new object, extension pages
  can still access settings synchronously.
  - This is especially important to options.html and popup.html; they
    will not work without it.

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