Forking a Git repository if personal quota is 0
On foss.heptapod.net and heptapod.host, most users don't have the rights to create projects in their personal namespaces. In the former, that is because we want to control what people upload on a very public instance. In the latter, because invoicing is tied to top-level (actual) groups.
But the Git fork page still proposes the personal namespace to host the fork. Similarly to what has already been done with creation / import pages, we must exclude it, and perhaps even add a message urging users to make personal subgroups of their top-level / company groups.