Investigate fork merge request collaboration

According to https://gitlab.eclipse.org/help/user/project/merge_requests/allow_collaboration:

When a user opens a merge request from a fork, they are given the option to allow upstream members to collaborate with them on the source branch. This allows the members of the upstream project to make small fixes or rebase branches before merging, reducing the back and forth of accepting external contributions.

This feature is available for merge requests across forked projects that are publicly accessible.

When enabled for a merge request, members with merge access to the target branch of the project is granted write permissions to the source branch of the merge request.

We should try this out. If it works well, this could replace the more complicated approach of #27 (closed), or be an alternative for it. It will likely not solve the situations where multiple contributors want to work with the project committers on a contribution.