Co-authoring metric

[LucPrestin]

As a team coach, I'd like to see a summary of the co-authoring activities of my team in given sprint(s), e.g. only the last, or all available ones (similar to the results of #18), so that I can see whether team members used the functionality and indicated when they worked together. I would like to have this information as a fraction of co_author_commits/team_commits.

I would also appreciate an indication of how many distinct "co-authoring teams" there were in a sprint, so I have an indication of how often teams were switched and knowledge sharing happened.

[chrisma]

My intuitive idea for a visualization of the co-authored commit percentages would be a stacked bar chart of co-authored and non-co-authored commits.

[chrisma]

Explanation for this metric should contain a link to how co-authoring works on GH