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/sass/layout/_headers.scss

@@ -1,7 +1,7 @@
 @charset 'UTF-8';
 
 .channel-header {
-    @include flex(0 0 56px);
+    @include flex(0 0 57px);
     border-left: none;
     font-size: 14px;
     line-height: 56px;