Adapt internal differences tool for external use

[bhauer]

The community would probably find some value in a tool that allows visualizing the differences between two runs. This issue concerns adapting our internal in-browser differences tool for external use.

Presently, the internal tool provides a bare-bones user interface to paste two results.json files into text areas and then visualize the differences as an HTML table. This could probably be retained mostly as-is, since I doubt anyone using this tool would care that the UI is minimalist. But it would be more immediately usable by the external audience if it were adapted to optionally parse two runs' hashes from the URL and load the results for those two runs from tfb-status (while still providing the means to manually copy and paste results data).

The tfb-status list of runs could then be improved to allow the user to select two runs and then click "diff" to navigate to the diff tool.

[lauxjpn]

I think there is a lot of merit to this.