Minor improvements on the admin-ui login page.

- Correct the bug with browser autofill (This bug is due to a known bug of angularjs https://github.com/angular/angular.js/issues/1460, but as the correction is a little bit too much code to add, I prefer a less verbose solution found there: http://victorblog.com/2014/01/12/fixing-autocomplete-autofill-on-angularjs-form-submit/).
- Default focus on the username input (using the html5 attribute autofocus).
- Change login button type to "submit" in order to be able to validate the form with the enter key.

(I didn't knew if I needed to add an issue, and submit the PR for the related issue ?!)