Fix google authentication back-out message (#1116)

![croppercapture 3](https://cloud.githubusercontent.com/assets/1271535/3487219/b206c084-046d-11e4-9a57-f8e6a9e13005.jpg)

It does pretty much the same thing, just using the extension method in https://github.com/JabbR/JabbR/pull/1139/files#diff-6f816fea17915437125c0374c9213f16R58 which ensures that the username contains only jabbr-safe characters