[admin] Only organizationuser objects which have admin=True should be able to manage their organization in the admin

It could happen that a user which has is_staff set to True becomes member of another organization because he wants to use some service (implemented in other openwisp modules) of that organization as a user, but at the moment we can’t let that happen because in the moment he become member of that organization he will also be able to edit the objects of that organization from the admin.

To solve this problem, we should change the code so that only OrganizationUser objects which are both member of an organization and have is_admin set to True are able to see and edit the objects of that organization.

When OrganizationUser.is_admin is set to False the user won’t be allowed to see anything related to that organization in the admin.

[nemesisdesign]

@OltarzewskiK up to you, there are no requirements in open source, contributors contribute when they can. If someone else comes up and can finish this issue before you do we shouldn’t prevent them from doing so, because it’s more important to fix the issue than who fixes it, I’ll send you some suggestions via PM

[strang1ato]

@nemesisdesign Are you sure that I have to change this method: https://github.com/openwisp/openwisp-users/blob/master/openwisp_users/admin.py#L195-L213 the class where it is referring to is I think for Usernot OrganizationUser, so have I to create new class in the admin?