Apply the app group name in app extension

(merge https://github.com/vector-im/riot-ios/pull/1440)

RiotShareExtension/ViewController/ShareViewController.m

@@ -63,6 +63,9 @@
 
 - (void)prepareSession
 {
+    // Apply the application group
+    [MXKAppSettings standardAppSettings].applicationGroup = @"group.im.vector";
+    
     // Prepare account manager
     MXKAccountManager *accountManager = [MXKAccountManager sharedManager];