Write relative path to MathJax when mathjax = "local"

The `mathjax = local` argument currently writes an absolute path into the document. This change makes that path relative, so the document can be relocated, and/or served over HTTP successfully. 

This change, in combination with https://github.com/rstudio/rstudio/commit/33cc1d0e4cf487591ba54905f08a43e15f3b266e, serves the local (patched) version of Mathjax inside Shiny documents, so math content can be rendered without timeouts on Qt platforms.