PLT-2786 Open Account Settings/Recent Mentions Shortcuts (#2957)

* Account settings shortcut added

* Added constants

* Added recent mentions shortcut

webapp/utils/constants.jsx

@@ -549,7 +549,9 @@ export default {
         ESCAPE: 27,
         SPACE: 32,
         TAB: 9,
-        U: 85
+        U: 85,
+        A: 65,
+        M: 77
     },
     CODE_PREVIEW_MAX_FILE_SIZE: 500000, // 500 KB
     HighlightedLanguages: {