Allow article lookup by ID without the blog_id requirement

As with variants, blog articles can be retrieved without needing to specify a blog_id. For example...

**This ...**
```
/admin/articles/8522335.json
```
**... is the same as this ...**
```
/admin/blogs/2907647/articles/8522335.json
```
Simply duplicated the solution already implemented for variants. Added _prefix classmethod to Article resource handling blog_id presence or absence same as the Variant resource does for product_id.