Show nicer metrics of audit progress

[ranweiler]

Right now, in the audit view, we display the reciprocal of risk limit, and the actual test statistic for the sequential probability ratio test. Instead (or perhaps, additionally), show the number of ballots that still need to be sampled at current discrepancy level. Also, indicate when a particular contest outcome is confirmed at the configured risk limit.

[nealmcb]

Showing the "risk level achieved" is how we've been describing it in what we've sent Colorado.

[ranweiler]

Closed by #67. Added #68 for future work.