GitLab now uses a `/-/` component between the `group/project` part of
the URL and the `{issues,merge_requests,tree}` part so that it can
support `group/subgroup/project` with arbitrary depth.

GitHub's link to the contribution guidelines in `CONTRIBUTING.rst`
is not prominent enough to be reliably noticed and followed by
new contributors.  Add a PR template to make the link more visible.