Add statistics for external vs internal (The Qt Company) contributors

We currently have a graph showing "Commits per email domain" statistics.

It would be great to additionally have a graph showing a split between total external contributors vs ones from The Qt Company. The X axis can count on month or even quarter basis so that it would be rather a long-term snapshot and not a daily "tracking".

Such a new graph should better visualise the involvement of community, since the number of commits is certainly higher for full-time developers, but on the other side a large number of people in the community contributing to Qt on regular basis is a better community metrics than a number of commits for a single party.