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.
Hmm, maybe a class name would be best in this case. But I fear we are drowning in CSS hidden class names... don't we already have `.hidden` and `.visually-hidden`?

I look in sandstone and see this:

    .hidden {
        .visually-hidden();
    }

Which I find itself confusing at best :confused: