[Files] Copy updates and file picker single select (#144398)

* implement copy updates

* move rename file and update empty state copy

* update title to respect multi select vs single select

* added test for single select behaviour

* implement single select behaviour

* introduce and use multiple prop, default true

* pass multiple flag to state

* pass multiple upload flag to UI

* added single select story

* update files example to still support multiple select

* uploadMultiple -> selectMultiple

* remove use of non-existent i18n

* update filepicker react component tests

examples/files_example/public/components/file_picker.tsx

@@ -27,6 +27,7 @@ export const MyFilePicker: FunctionComponent<Props> = ({ onClose, onDone, onUplo
       onDone={onDone}
       onUpload={(n) => onUpload(n.map(({ id }) => id))}
       pageSize={50}
+      multiple
     />
   );
 };