Redesign Firefox Sync page for Bug 1046251

- Conditional contents depending on browser type/version
- New content/design only applies to en-US until other locales opt-in

https://bugzilla.mozilla.org/show_bug.cgi?id=1046251

The /firefox/sync/ page now has six variations (in en-US and any other locale that opts-in):

1. Firefox 31+ (signed-in to Sync) <-- default
2. Firefox 31+ (signed-out of Sync)
3. Firefox 29 or 30
4. Firefox 28 or older
5. Firefox for Android
6. Not Firefox (any other browser)

The content for each variation can be found on the copy bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1030427
Since much of the logic in the `firefox_sync` JS bundle is now mainly for the new improved Sync page, most of it doesn't apply to this old template. Nothing breaks, but I wonder if it's worth giving this page its own bundle to reduce risk of introducing accidental bugs over time?