Content libraries paging

`library` is always a root xblock, so this whole method can be reduced to:

```python
def author_view(self, context):
    fragment = Fragment()
    self.render_children(context, fragment, can_reorder=False, can_add=True)
    return fragment
```

You could even combine `render_children` and `author_view` into one `author_view` method. I would also suggest removing the `can_reorder` code from these methods completely, since we won't have manual ordering in libraries (just always pass `can_reorder: False` into the context).