Add login placeholder logic to OnlineOverlay

A perfect implementation of this would probably leave the filter/header
content visible, but that requires some re-thinking and restructuring to
how the content is displayed in these overlays (ie. the header
component shouldn't be inside the `ScrollContainer` as it is fixed).

Supersedes and closes #10774.
Closes #933.
Addresses most pieces of #7417.

osu.Game/Overlays/ChangelogOverlay.cs

@@ -30,7 +30,7 @@ namespace osu.Game.Overlays
         protected List<APIUpdateStream> Streams;
 
         public ChangelogOverlay()
-            : base(OverlayColourScheme.Purple)
+            : base(OverlayColourScheme.Purple, false)
         {
         }