PLT-2755 Shortcut to upload file (#2848)

* Added file upload shortcuts (CTRL+U and CMD+U)

* Added KeyCode for U

webapp/utils/constants.jsx

@@ -544,7 +544,8 @@ export default {
         ENTER: 13,
         ESCAPE: 27,
         SPACE: 32,
-        TAB: 9
+        TAB: 9,
+        U: 85
     },
     CODE_PREVIEW_MAX_FILE_SIZE: 500000, // 500 KB
     HighlightedLanguages: {