Inconsistent value display style on history page

Values drawn on the history page (e.g. in the tooltip or the sidebar) do not match the style that is used in the test result tables.

E.g. `datetime` values are printed in the table in a human-friendly format `H:MM:SS.SSSSSS`, while in the sidebar of the history page the ISO 8601 string representation is used and the tooltip of the chart just prints the numerical value in seconds.

The underlying cause for this issue is that the representation for the results table is generated on the server-side by Django but the representations on the history page are generated on the client-side in Javascript, where we do currently lack support for human friendly representations.

The best approach would probably be to write a bunch of client side functions to generate the desired representations and use those everywhere consistently.