Firefox pages JS should consider ESR builds up to date bug 828064

Refactor of pull request #585 to bring JS up to date (Bug 828064).

This refactor includes suggestions based on the original code review, along with a few additions:

* Moving page specific inline JS to external files.
* Moving `isFirefox()` `isFirefoxUpToDate()` functions to `global.js`.
* Use of data attributes for Firefox and ESR version.
* L10n for heading and sub-heading in `update.html`.
* Additional ESR check in `devices.js` and `devices.html`

@pmclanahan @Osmose r?