SiteConfig load/save with ID in CMS

Allow loading a SiteConfig by ID (by specifying $tree_class),
and pass the ID through with the form data. Unifies processing
with SiteTree, and allows the Translatable module
to use the same logic for interacting with the load/save process.

code/controllers/CMSSettingsController.php

@@ -5,6 +5,7 @@ class CMSSettingsController extends LeftAndMain {
 	static $url_rule = '/$Action/$ID/$OtherID';
 	static $menu_priority = -1;
 	static $menu_title = 'Settings';
+	static $tree_class = 'SiteConfig';
 
 	public function getResponseNegotiator() {
 		$neg = parent::getResponseNegotiator();

code/model/SiteConfig.php

@@ -65,7 +65,8 @@ class SiteConfig extends DataObject implements PermissionProvider {
 					$topLevelCreatorsGroupsField = ListboxField::create("CreateTopLevelGroups", _t('SiteTree.TOPLEVELCREATORGROUPS', "Top level creators"))
 						->setMultiple(true)->setSource($groupsMap)
 				)
-			)
+			),
+			new HiddenField('ID')
 		);
 
 		$themeDropdownField->setEmptyString(_t('SiteConfig.DEFAULTTHEME', '(Use default theme)'));