Changed text submission editing textarea width

Changed text submission editing textarea width so that it scales
according to the user's page size, avoiding waste of space on larger
monitors ([Before](http://i.imgur.com/xAF2ymz.png) and
[After](http://i.imgur.com/HqHzwHD.png)) and allowing to scale down for
smaller page sizes ([Before](http://i.imgur.com/EysQ3M9.png) and
[After](http://i.imgur.com/TNqX151.png))