Fix timeline jump button having background when button shapes accessibility setting is on

Tusker/Screens/Timeline/TimelineJumpButton.swift

@@ -20,6 +20,8 @@ class TimelineJumpButton: UIView {
         var config = UIButton.Configuration.plain()
         config.image = UIImage(systemName: "arrow.up")
         config.contentInsets = .zero
+        // We don't want a background for this button, even when accessibility button shapes are enabled, because it's in the navbar.
+        config.background.backgroundColor = .clear
         return UIButton(configuration: config)
     }()