FIX: quote reply button position was wrong when there is a custom header

This fixes an issue reported by @SamSaffron [on meta](http://meta.discourse.org/t/quote-reply-gets-in-the-way/1495/60) that when there was a custom header, the positioning of the quote reply button was wrong.

Browsers tested:
- [x] IE 10 (Windows 8)
- [x] Chrome 26 (Windows 8)
- [x] FF 20 (Windows 8)
- [x] Safari 6 (OS X 10.8.3)
- [x] Chrome 25 (iOS 6.1.3)
- [x] Safari (iOS 6.1.3)