Project, groups, user improvements

[faymard]

This pull request introduces a batch of changes/improvements I've done over the past months to fit requirements for a company project.

It requires a huge rebasing process as I've not followed upstream during the whole period (shame on me). I think some of the additions I've made have been done and merged by other contributors in the meantime.

I guess some test methods also have to be added.

There is no emergency on my side to have this PR merged, so please do not spend time on rebasing, I will do it myself when I can. If there is a need for you or some users, then I'll try to do this soon.