Add ability to toggle discord rich presence

There are 3 modes: enabled, limited, and disabled.

The limited mode hides identifiable information such as username, rank, and
(if participating in one) multiplayer lobby name.

osu.Game/Overlays/Settings/Sections/OnlineSection.cs

@@ -20,7 +20,8 @@ namespace osu.Game.Overlays.Settings.Sections
         {
             Children = new Drawable[]
             {
-                new WebSettings()
+                new WebSettings(),
+                new IntegrationSettings()
             };
         }
     }