Clean up logged-out change email form

Change email form was missing default Bootstrap classes, meaning disabled fields didn’t look disabled, labels weren’t above fields and fields were the wrong size.

* Add bootstrap classes
* Keep page title and h1 consistent
* Move warning message out of well to avoid nesting
* Stop nested labels around passphrase field

Before:
![screen shot 2014-10-06 at 11 13 23](https://cloud.githubusercontent.com/assets/319055/4524589/5776efa0-4d46-11e4-8884-47b67ce1aedf.png)

After:
![screen shot 2014-10-06 at 11 45 13](https://cloud.githubusercontent.com/assets/319055/4524592/5cb17c56-4d46-11e4-8160-f20c249c0f2b.png)