github: add support for Hacktoberfest using labels

github: add support for Hacktoberfest using labels

Automatically add hacktoberfest-accepted label to PRs opened between
September 30th and November 1st once a commit with a close reference
to it is pushed onto the master branch.

With this workflow we can participate in Hacktoberfest while not
relying on GitHub to identify PRs as merged due to our rebasing.

Requires hacktoberfest-accepted labels to exist for PRs on the
participating repository. Also requires hacktoberfest topic on
the participating repository to avoid applying to forked repos.

Reviewed-by: Daniel Stenberg
Fixes #7865
Closes #7897