WIP - Bug 1082444 - Use web activities to display FxA terms/privacy page...

...s.

  * We need to dismiss FxA when the terms/privacy pages are displayed, which is
    the same behavior as needed to complete the password reset, so we adopt the
    same approach. This has the nice side effect of switching from the
    non-standard EntrySheet to standards track web activities, and paves the
    way for FxA to be removed from the system app someday.
  * Moved the terms/privacy URLs into settings, so that QA can test on staging
    environments, if needed.

  Still TODO: update tests; make sure it works.
In retrospect, I think `identity.fxaccounts.url` would have been the better way to organize the namespace.

But I don't want to create a backwards incompatibility by reversing it for the reset-password url, and this is readable enough.