Add HREF hints to JSON and let vis.js use them

vis.js has old assumptions about links in table headers (builds, tests, hosts).

vis.py commit adds hints to the JSON file which would simplify vis.js somewhat.

vis.js change is untested.

In practice, the vis.py hinting has to be made more specific to support the lack of autoindexing and index.html in CI_DRM and IGT result directories, but this would be a good start.