Github linkify for CMSSW

Makes C++ and Python files in CMSSW on Github much easier to navigate by turning filepaths into links! No more copy-and-pasting! Also works on forks.

C++: convert "#include" statements into links. Also adds a link to the source file if it exists.

Python: convert "import X" and "process.load('X')" statements into links.

NOTE: there is a current known issue, whereby you must sometimes reload the page to get links to appear.