Use content-box for text inputs

It's normally the default anyway but it's becoming very popular to set
`* { box-sizing: content-box; }` because it fixes so many annoying things.
Unfortunately, it can cause a problem when height and top or bottom
padding are set. In the case of this element, if border-box has been
set, we will only have 5px of vertical space left. So make sure it's
using content-box.