Use absolute URL to open rendered PDFs

This fixes a bug wherein PDFs were not displayed when RStudio Server is serving pages behind a path other than `/`. Formerly, the PDF.js viewer was loading a server-relative URL (e.g. `/rmd_output/1/`); this change makes that URL absolute (e.g. when RStudio Server is running behind `/rstudio/`, `http://server/rstudio/rmd_output/1`).