Fix POST /internal/allowed to cope with gitlab-shell v4.0.0 project paths

gitlab-shell v3.6.6 would give project paths like so:

* namespace/project

gitlab-shell v4.0.0 can give project paths like so:

* /namespace1/namespace2/project
* /namespace/project
* /path/to/repository/storage/namespace1/namespace2/project
* /path/to/repository/storage/namespace/project