loginDialog: drop padding between buttons and entry

Now that the closed session list preallocates space for its
open self, there is a lot of padding between the entry and
buttons.

This commit helps alleviate the padding problem by getting rid
of the large top padding set above the login buttons.

data/theme/gnome-shell.css

@@ -2240,6 +2240,10 @@ StScrollBar StButton#vhandle:active {
     min-width: 350px;
 }
 
+.login-dialog-button-box {
+    spacing: 21px;
+}
+
 .login-dialog-prompt-login-hint-message {
     font-size: 10.5pt;
 }

js/ui/auth/loginDialog.js

@@ -180,7 +180,7 @@ const LoginDialog = new Lang.Class({
                               y_fill: true,
                               x_align: St.Align.START });
 
-        this._buttonBox = new St.BoxLayout({ style_class: 'modal-dialog-button-box',
+        this._buttonBox = new St.BoxLayout({ style_class: 'login-dialog-button-box',
                                              vertical: false });
         this._promptBox.add(this._buttonBox,
                             { expand:  true,