Fix loginbox username max width at 300px.

Hi Bas,

Fixes this:

![2014-05-25 14_51_39-moodle 27_ log in to the site](https://cloud.githubusercontent.com/assets/1058419/3077118/b20b7832-e414-11e3-8826-8261890f7a58.png)

Cheers,

Gareth