moved the id to the AbstractPage class itself

When a Page is exposed via the ApiBundle (will be released later on), the NodeVersion->getId() is used as the unique Page identifier. To achieve this, we need to exclude the id from the Page class itself, and replace it with the id of the NodeVersion. To exclude the id from the Page class, we need to configure the SerializerBundle via a yml config file were we exclude the id from all AbstractPage classes. If we would exclude the id from the AbstractEntity, all exposed objects would not have an id.