Improve user experience editing USX text

Instead of a checkbox, make the "Edit Previous Text" option a link that
fills the box with the previous text. In all circumstances, the text
entered into the box will become the new text when the user hits Submit,
whether it was edited by hand or whether a new USX was uploaded.

This should fix the problem where before, the user had to check the "I
know what I'm doing (dangerous)" checkbox before even uploading a
brand-new text (which is not actually dangerous).