import: Convert arrow function to regular function

This fixes a bug introduced in commit
b711ff6acf. Some time between when that
commit was originally written and when it was merged a round of
linting had converted the function from a regular function to an arrow
function because `this` was never in the body of the function. When I
rebased the commit, which introduced `this` to the body, I didn't
catch the error.

src/static/js/pad_impexp.js

@@ -42,7 +42,7 @@ const padimpexp = (() => {
     $('#importmessagefail').fadeOut('fast');
   };
 
-  const fileInputSubmit = (e) => {
+  const fileInputSubmit = function (e) {
     e.preventDefault();
     $('#importmessagefail').fadeOut('fast');
     if (!window.confirm(html10n.get('pad.impexp.confirmimport'))) return;