Add "contributed" boolean property to CLA.json

[pjcozzi]

Sometimes we receive a CLA, but never receive a contribution. It is nice to track that.

We also receive a corporate CLA with a list of potential contributors in the Schedule A section, but then those contributors are warned on their first PR that we do not have a CLA for them. It would be nice to add them as a contributed : false. For bonus, the concierge could automatically change it to true when their first PR is merged. The concierge would need write access.

To be extra cool, instead of a boolean this could be number of commits or PRs, then the concierge can "welcome back" on their 2nd PR and celebrate milestone PRs like the 10th, 25th, etc.