fix(sync): show Sync brand name when signin up/in for Sync

Fixes #1339.

WIP, we might need string changes.
As @zaach points out in https://github.com/mozilla/fxa-content-server/issues/1339#issuecomment-47986909, wrapping part of the sentence in a span to affect layout may cause issues in some locales where the sentence structure causes "to continue to %(serviceName)s" to appear before "Create a Firefox Account". @mathjazz - what is the best practice here? The goal is to make this look something like: https://cloud.githubusercontent.com/assets/68213/3448014/89a995a6-0154-11e4-8e9f-5d6163ad26cf.png