Add GitHub activity to user profile

Report https://news.ycombinator.com/item?id=11089431

Would allow users that use both GitLab and GitHub to have a complete profile in one place.

This would reduce the friction to switch

1. For all users that have a GitHub account connected
2. Make sure to disclose in terms
3. Offer a way to turn it off

/cc @dzaporozhets @JobV