Fix Single File Template Asset Inclusion

Fix single file custom template inclusion

- Pass `$this->render` ( FILENAME_BLOCK_VIEW )  into
  `BlockView::getBlockURL()` on single file custom templates to avoid
  attempting to include root level assets like view.css that do not
  exist and generate 404's
- This happens because `BlockView::getBlockURL()` checks for the
  existance of the directory and because there is a custom template in
  site root, it believes the base block is being overridden
- Passing in `$this->render`, essentionally 'view.php', to
  `BlockView::getBlockURL()` will ignore the site root block directory
  unless it contains 'view.php'.
- This approach is chosen over checking if directory is empty for
  performance concerns. But that is another option.
- Possible side effect is that if a user was attempting to override
  view.css in the core block, the template would go to the core view.css
  not the site root

http://www.concrete5.org/developers/bugs/5-6-1-2/404-when-using-custom-template-for-page-list-block/
http://www.concrete5.org/developers/bugs/5-5-2/blockviewtemplategetbaseurl-returns-inconsistent-results/