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.
So, we are reusing this general approach (grab the URL via the pref) for the other two URLs.

We are also lazily fetching the URLs. Lazy is good.

TODO: let's cache the URL on the object via something like: `if (!this.termsURL) { lazy load it }`. That also gives us a way to remove the duplication between the terms and privacy functions.