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.
non-visible content that has focusable elements only confuses keyboard-only users, I would say use a `.hidden` class with `display:none;` or `hidden` attribute to hide them.