fixed vertical scroll for modal dialog

src/app/files/file-explorer.js

@@ -319,7 +319,7 @@ fileExplorer.prototype.init = function () {
 
 fileExplorer.prototype.publishToGist = function () {
   modalDialogCustom.confirm(
-    null,
+    'Create a public gist',
     'Are you sure you want to publish all your files anonymously as a public gist on github.com?',
     () => { this.toGist() }
   )

src/app/ui/styles/modaldialog-styles.js

@@ -13,6 +13,8 @@ var css = csjs`
   }
   .modalBody {
     word-break: break-word;
+    overflow-y: auto;
+    max-height: 600px;
   }
   .modalFooterOk {
   }