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 would maybe edge toward keeping two CSS classes, but make each a bit more clear in what it does: 
* `.hidden` (display: none), content hidden both visually and from screen readers.
* `.visually-hidden`, content hidden visually but *not* hidden from screen readers