Explicitly insert bibliography (greenelab#69)

Enables content that follows the bibliography

From https://github.com/jgm/pandoc-citeproc/blame/5a7f26b61c8577916093851cdeb31fa9a198edcb/man/pandoc-citeproc.1.md#L126-L128

> a bibliography will be inserted into each Div element with
id refs. If there is no such Div, one will be created at the
end of the document.

content/99.back-matter.md

@@ -1 +1,4 @@
 ## References {.page_break_before}
+
+<!-- Explicitly insert bibliography here -->
+<div id="refs"></div>