Support for projects hierarchy

Wouldn't it be nice having some kind of projects hierarchy in GrimoireLab?

Currently GrimoireLab only supports 1 level hierarchy according to [projects.json example](https://github.com/chaoss/grimoirelab-mordred/blob/master/docker/example_conf/projects.json) and [the existing documentation](https://grimoirelab.gitbooks.io/tutorial/content/mordred/the-projects-file.html). So we have `Project > Repositories`

In [Bestiary](https://github.com/chaoss/grimoirelab-bestiary) is being introduced the Ecosystem level, so 2 levels are gonna be supported (sort of): `Ecosytem > Project > Repositories`

Is it gonna support `Ecosystem > Project > ... > Project > Repositories`? Do we need that deepth? This might be also related with #71

I would like also knowing if the community would agree on the following assumptions:
* Do we expect that some same repositories might be in different projects or ecosystems? I would say "no, by design".
* Might some projects be under no specific ecosystem? I would say "no, by design".
* Might some repositories be under no specific project? I would say "no, by design".
* Project names might be not unique. For example, several ecosystems might have a "Documentation" project. This should not be an issue.

Thanks for the comments.

Start with the projects.json example and the existing Mordred projects-file documentation linked in the issue, then compare the Ecosystem model being introduced in Bestiary and related issue #71. Done would require an agreed hierarchy depth and documented decisions on repository ownership, ecosystem membership, and project-name uniqueness before implementation scope can be defined.

Written by the indexing model from the issue text.