fix "restore default" button, should be visible only when field has custom value

added missing validations for setup script
added two field descriptions
added SkipLockedTables to user preferences

libraries/config/user_preferences.forms.php

@@ -17,7 +17,8 @@ $forms['Features']['General'] = array(
     'NaturalOrder',
     'InitialSlidersState',
     'ErrorIconic',
-    'ReplaceHelpImg');
+    'ReplaceHelpImg',
+    'SkipLockedTables');
 $forms['Features']['Text_fields'] = array(
     'CharEditing',
     'CharTextareaCols',