Show back button when spectating

Avoids getting stuck at some screens.
It's a bit ugly having the back button visible like this,
but is the best approach we have for now.

osu.Game/Screens/Play/SpectatorPlayer.cs

@@ -25,6 +25,8 @@ namespace osu.Game.Screens.Play
 
         private readonly Score score;
 
+        public override bool AllowBackButton => true;
+
         protected override bool CheckModsAllowFailure()
         {
             if (!allowFail)