authPrompt: Fade out cancel button after verification

Right now we make the cancel button non-reactive when
verification is complete, but it still disrupts the
login transition because in many cases most of the other
screen elements have been faded out.

This commit fades it out as well.

js/gdm/authPrompt.js

@@ -536,6 +536,11 @@ export const AuthPrompt = GObject.registerClass({
         this.verificationStatus = AuthPromptStatus.VERIFICATION_SUCCEEDED;
         this.cancelButton.reactive = false;
         this.cancelButton.can_focus = false;
+        this.cancelButton.ease({
+            opacity: 0,
+            duration: MESSAGE_FADE_OUT_ANIMATION_TIME,
+            mode: Clutter.AnimationMode.EASE_OUT_QUAD,
+        });
     }
 
     _onReset() {