Add team ranking metrics for speakers

[tienne-B]

Some tournaments really try to avoid ties in the speaker standings, and for that, they often use team metrics, such as "team points", "total speaks", "total firsts", etc. We already have a "team points" metric for speakers, but could make metrics in https://github.com/TabbycatDebate/tabbycat/blob/develop/tabbycat/standings/speakers.py for the others.