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/, /firefox/partners,
  /firefox/os/devices/, and /firefox/channel/ to use
  newer pager features.
- Re-structured hider CSS - .visually-hidden is now a proper
  class (not just a mixin), and .hidden sets display: none;
Merge pull request #2106 from jpetto/bug-966702-improve-mozilla-pager-js

Improve mozilla-pager.js. Bug 966702.