Add location option to allow rendering an L&L layout before the closing `head` or `body` tags

Requested by Ben in an internal discussion. L&L layouts could theoretically be used as a replacement for those single-purpose plugins that insert headers and footers. To do this, there would need to be an option in the “location” menu of an L&L layout that would allow selecting whether the template should be loaded before the closing head tag or before the closing body tag.