simplify mobile worker username display

get rid of `@<domain>.commcarehq.org`

**Before**
![before](https://f.cloud.github.com/assets/137212/1229454/d2add922-27b8-11e3-93b8-303dbf9287ce.jpg)


**After**
![after](https://f.cloud.github.com/assets/137212/1229456/d810c23a-27b8-11e3-8fd8-828287f552fa.jpg)


(But don't worry, it's still the full form when a mobile worker logs in to CloudCare)
![top](https://f.cloud.github.com/assets/137212/1229457/e1874096-27b8-11e3-87b5-78a67353408f.png)