Fix scroll position when invoking reader on a block.

The problem was that the block is not folderish and therefore was not added in the URL automatically.
The fix is to use the base-url from the <base> tag.

The result was that even though that it did always scroll to the chapter when invoking the view, even when the view was invoked on a block. When having large blocks above the block was not visible at all.

For example the keywords tab links to the reader view directly on a block.

@maethu might take a look?