polkitAgent: Expand password entry to the full width

Due to an oversight, the width of the password entry is currently
determined by the length of the message description. Fix the flags
so that the entry spans the entire width of the dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=684810

js/ui/components/polkitAgent.js

@@ -49,7 +49,7 @@ const AuthenticationDialog = new Lang.Class({
         let messageBox = new St.BoxLayout({ style_class: 'prompt-dialog-message-layout',
                                             vertical: true });
         mainContentBox.add(messageBox,
-                           { y_align: St.Align.START });
+                           { expand: true, y_align: St.Align.START });
 
         this._subjectLabel = new St.Label({ style_class: 'prompt-dialog-headline',
                                             text: _("Authentication Required") });