It's a teamcity plugin.
It superseeds https://github.com/leipreachan/tc-better-reports browser extension. It was good, simple, nice, fast, had some funny bugs.. and didn't work in Safari.
I'll get to know how to make it better (sooner or later)
To build the plugin use:
mvn compile package
and look for a new zip file in better-test-reports-java/target/ directory