Hotfix: PLT-4779 Fixing scrolling on loading a channel and missing DM channel headers (#4584)

* Added all parameters to call tracker ids when getting profiles

* Changed channel header rendering to not depend on knowing all users in a DM channel

* Added comment about a race condition in UserActions.populateDMChannelsWithProfiles

* Added a fixed-height placeholder for the ChannelHeader when its state isn't valid for rendering

webapp/actions/user_actions.jsx

@@ -135,6 +135,7 @@ function populateDMChannelsWithProfiles(userIds) {
     const currentUserId = UserStore.getCurrentId();
 
     for (let i = 0; i < userIds.length; i++) {
+        // TODO There's a race condition here for DM channels if those channels aren't loaded yet
         const channelName = getDirectChannelName(currentUserId, userIds[i]);
         const channel = ChannelStore.getByName(channelName);
         if (channel) {