Full user management support for sub-projects

[rdimitrov]

This issue is a placeholder for fixing the missing items for supporting all user management features in sub-projects.

Currently this is partially supported, i.e. one user can invite another user to their sub-project, but it has not been properly tested and synced with the FE and design teams.

For example, listing role assignments may have to be updated, so when we query for the role assignments of the parent project we may want to include the role assignments of all child sub-projects. This is needed because someone can have access to a sub-project but no necessarily have access to its parent project. Nevertheless the admin of the parent project should be able to get this overview.

[evankanderson]

Sub-projects is an epic in #6253 , we'll need to collect all those issues to tackle together.

[evankanderson]

Leaving this parked, will create an epic for hierarchical multitenancy to parent this and other concerns under, so we can track them all.