Improvements to user mgmt form.

- Display username of a currently edited user
- Remove the alert when a new username is typed
- Store typed username in app model so that it's not deleted when the alert pops up (allows to correct typos in the username)

Fixes #423 

![image](https://cloud.githubusercontent.com/assets/2522010/4795230/c9b94152-5dfa-11e4-86ed-4c926a15732b.png)