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.
Even though one is a mixin and one a class, `.visually-hidden` & `.hidden` are indeed confusing in tandem. It would be nice to maybe just make `.visually-hidden` a class and audit all CSS to use that and do away with the `.hidden` class. Would like to hear from @craigcook here, too.