authPrompt: allow cancellation before verification starts

The user should be allowed to cancel if verification hasn't
started yet and they're typing in their username. This
commit changes the authPrompt cancel function to not
ignore such requests.

https://bugzilla.gnome.org/show_bug.cgi?id=752739

js/gdm/authPrompt.js

@@ -501,7 +501,7 @@ const AuthPrompt = new Lang.Class({
     },
 
     cancel: function() {
-        if (this.verificationStatus == AuthPromptStatus.NOT_VERIFYING || this.verificationStatus == AuthPromptStatus.VERIFICATION_SUCCEEDED) {
+        if (this.verificationStatus == AuthPromptStatus.VERIFICATION_SUCCEEDED) {
             return;
         }
         this.reset();