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
> On second thought, is there any reason for this not to be a real download button with simple=true?

I did try that, but the design/copy calls for different text on the button ("Download Firefox"), while the simple=true variation of the download button has the text "Free Download". I wondered about changing the download buttons to allow an alternate string to be passed in, but I wasn't sure we should be complicating the buttons further.