Add activity summary

[pmonks]

I think we need a short activity summary on each card. Right now there's a lot of data about each activity, but no explanation of what each one does.

[pmonks]

Ideally this would be the same as the GitHub "one line description", but clearly we can't actually use that, since activities may have any number of GitHub repos (including zero). So I think this will need to be stored as metadata of each project, and generated by the metadata tool.