Disable spellcheck and autocorrect for username field in admin page

closes #14855

## before
![Screen_Shot_2016-04-19_at_9.25.12_AM](/uploads/19387c4a11a879e9034a2179e932436e/Screen_Shot_2016-04-19_at_9.25.12_AM.png)

## after
![Screen_Shot_2016-04-19_at_9.25.47_AM](/uploads/60f4eba6a97201333dc64e46d625aac2/Screen_Shot_2016-04-19_at_9.25.47_AM.png)

See merge request !3730