Improve mozilla-pager.js. Bug 966702.

- Remove unused 'pager' JS bundle.
- Add docs pages for mozilla-pager (Bug 1007717).
- Add mozilla-pager.js demo page to styleguide (dev only).
- Add ARIA support for mozilla-pager.
- Add ability to create/destroy pagers.
- General performance/cleanup of mozilla-pager.js.
- Updated /firefox/desktop/tips/ and
  /firefox/channel/ to use newer pager features.
I'm unsure if we should be tying presentational styling to behaviour directly associated with WAI-ARIA this way. There may well be cases where we would want to apply the `aria-hidden` attribute but not have `display:none` applied for instance.

How about we use the HTML5 `hidden` attribute instead? http://blog.paciellogroup.com/2013/11/html5-accessibility-chops-hidden-aria-hidden-support/