improve project collaborator management

[haraldschilly]

There are two types of users: "owners" and "collaborators".

The goal of this enhancement is to tighten who can control the users of a project.

The first part of this is to manage ownership vs. collaborator types.

• owners can change a collaborator to an owner, or an owner to a collaborator
• it is not possible to remove an owner
• it is not possible to change the last owner to a collaborator (i.e. one owner must be the minimum)
• collaborators cannot change a user type (in particular, can't make oneself or others to be an owner)
• owners can step down as being an owner by changing their own type, if there is another owner

The second part:

Add a setting to projects, which is disabled by default: "only allow owners to manage collaborators". The disabled behavior is the current behavior.

If it is enabled, there is one additional constraint: collaborators cannot add or remove other collaborators – just like they cannot remove an owner in all the cases.