Fairness Dashboard: table should have the right performance and fairness metrics

[romanlutz]

It should by default show the chosen performance and fairness metrics. The fairness metric won't have individual values per group since it's an aggregate value. It may be nice to show relevant metrics, though, e.g. selection rate when the fairness metric is demographic parity.

There should be a description for how it calculates these metrics including which groups contributed min and max value.

#65 goes a step further by making the columns configurable but that's out of scope for this initial change.

[romanlutz]

Closing this since the metrics are shown. For insights and description we have #104