Add mentors as project collaborators

[matthewfeickert]

Using https://github.com/Samcoodess/reana-dms/settings/access add the project mentors (@matthewfeickert @kratsg @lukasheinrich @pibion) as project collaborators.

(I would normally assign @Samcoodess to this but I need collaborator access to do that)

[Samcoodess]

Invited all the mentors to be project collaborators.