contrib/gitdm: add a new interns group-map for GSoC/Outreachy work

It makes sense to put our various interns in a group so we can see the
overall impact of GSoC and Outreachy on the project.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Mahmoud Mandour <ma.mandourr@gmail.com>
Cc: Ahmed Karaman <ahmedkhaledkaraman@gmail.com>
Cc: César Belley <cesar.belley@lse.epita.fr>
Message-Id: <20210720232703.10650-19-alex.bennee@linaro.org>