Bug 1022900: Add 2nd table of downloads for ESR when available.

`firefox_details` has the ability to give you an `esr_next` build version.
Release management will be adding a "next" version to `product_details` and
we'll be relying on that.

This works now, but the current value for FIREFOX_ESR_NEXT is a version that has
no builds yet. So what I did to test w/ a 2nd table is the following:

1. Update product details: `./manage.py update_product_details`
2. Edit `lib/product_details_json/firefox_versions.json` changing 'FIREFOX_ESR_NEXT' to '30.0'
3. Profit!

@jpetto hit me up on IRC with further questions.