Linkify source code locations

[Vtec234]

It would be useful to have the Kevin frontend turn locations of source code in the build output into links pointing at the PR repo. To do this, it would have to find filenames in the output, check whether they correspond to sources (and not build artifacts, include locations, etc.) and then turn them into links. If there turns out to be too many, degrading browser performance, we could only turn locations in errors or warnings into links.