Migrate and manage access level according to Tails roles

On #27 it appears that the group/project structure that was chosen (for good reasons!) introduces a regression wrt. how we manage membership and access level on repositories and issues.

• specify our needs on the blueprint
• think about the priority of the corresponding implementation (it could be that it can wait post-migration; it could be that without this, the migration itself will be needlessly painful; I don't know at this point)
• implement something that matches the specs
• document how to use this implementation in wiki/src/contribute/working_together/GitLab.mdwn on the feature/15878-gitlab branch ("Access control" section)