PLT-4343 Fixes for mobile main menu (#4148)

* Fixed mobile app link in the main menu to be displayed on mobile browsers

* Fixed doubled up dividers in mobile menu

* Added scrolling to mobile main menu

webapp/utils/user_agent.jsx

@@ -77,8 +77,15 @@ export function isAndroidWeb() {
     return isAndroidChrome();
 }
 
+// Returns true if and only if the user is using a Mattermost mobile app. This will return false if the user is using the
+// web browser on a mobile device.
 export function isMobileApp() {
-    return isAndroid() || isIos();
+    return userAgent.indexOf('iPhone') !== -1 && userAgent.indexOf('Safari') === -1 && userAgent.indexOf('CriOS') === -1;
+}
+
+// Returns true if and only if the user is using Mattermost from either the mobile app or the web browser on a mobile device.
+export function isMobile() {
+    return isIos() || isAndroid();
 }
 
 export function isFirefox() {