PLT-3785 Re-enabled marking messages unread (#4179)

* Re-added ALT+Click to mark msgs unread, fixed mentions for DM

* Added string to /shortcuts

webapp/stores/channel_store.jsx

@@ -328,8 +328,9 @@ class ChannelStoreClass extends EventEmitter {
         let chMentionCount = chMember.mention_count;
         let chUnreadCount = ch.total_msg_count - chMember.msg_count;
 
+        // Temporary workaround for DM channels having wrong unread values
         if (ch.type === 'D') {
-            chMentionCount = chUnreadCount;
+            chMentionCount = 0;
         } else if (chMember.notify_props && chMember.notify_props.mark_unread === NotificationPrefs.MENTION) {
             chUnreadCount = 0;
         }