Add support for subgroups in personal namespace

Description

Right now, to hierarchically organize multiple personal projects you either have to create groups or use some sort of hierarchical naming scheme for your user projects. Both approaches are not ideal: With groups, you only have one additional hierarchy level. Using a custom naming scheme, e.g., publications-papers-paper1, still clutters your user projects page and results in very long repository names after cloning (or you have to accept that the top-level directory name on your disk does not match the repository name). Therefore, I propose to add a mechanism to allow the organization of user projects inside "directories" directly in GitLab.

My personal use case is to better organize my work-related projects inside GitLab. At least two large research institutions in Germany that I'm working with (RWTH Aachen University and Forschungszentrum Jülich) have set up GitLab instances that can be used by their members for free. I (and several of my colleagues) use this to store work-related projects, such as scientific publications, research data etc. However, due to the inability to add some sort of hierarchy, I'm starting to lose track of all projects I have and it is also difficult to browse for something that you know/feel exists but not exactly its name.

~"feature proposal"

Proposal

Add ability to hierarchically organize user projects by introducing (sub-)directories for each user. Optionally, extend this feature by adding subdirectories to groups as well.

Links / references

I've found (at least) two proposals that tackle similar issues: gitlab-ce#19860 and gitlab-ce#2772. However, they seem to be limited to either groups or to add projects within projects, which is not exactly what I am looking for.