add "new contributors issues" metric or similar

[ncclementi]

I noticed that not only the amount of issues opened have grown but that a significant amount of "new people" (basically excluding frequent contributors and maintainers) have opened issues, which is an indication of health of the project.

Options of implementation to discuss suggested by @lostmygithubaccount

Add a mapping of common contributors and add that:

• hardcode it (that's fine)
• use GH API to get the members of the Ibis project teams (also faulty, cuz e.g. NickCrews would be in there)
• use 2 but check for "voltrondata" in the GH profile (also faulty, many people don't do it)

Do analysis to check if it's the person's first issue

issues = issues.mutate(is_first_issue=<something>), probably some kinda row number shenanigans + case statement