Improve unsaved changes 2

Replaces !1147 (closed). This one does not contain the "save" button as "save and abort" was confusing and "save and continue" is hard to do. So basically, this is just a restyled version of the browser dialog.

We hope that using styling in line with the site rather than browser will change how users interact with the dialog. Since we are not really sure ourselves we keep the option of rolling this back in the future.