Add support for grouping repositories

[pedrolucasp]

Today we have a bug where if a repository is under a directory, we don't load it
correctly. As I wrote a patch to fix this, I started to wonder if adding support
for grouping repositories would not make more sense. I think this should be
discussed if we should infer if the repository path has a slash, we should group
it (cgit does this), or if we should add first class support to the project
group, with its title, description, etc. It would also make more sense
considering that smithy shows "Projects", and some are often made up of several
different repositories.