Project stats and metadata

Luis has requested we provide the ability to track and breakdown user contributions in a project.

We'd need to come up with a list of useful stats to track. Most or all of these could be reconstructed simply from Automerge history.

The issue names no files, tests, or entry points. Start by reviewing how project history is represented through Automerge, then define which contribution statistics and breakdowns are useful. Done requires an agreed list of stats and a concrete implementation scope.

Written by the indexing model from the issue text.