Add possibility to manage project role assignments without admin permission

[Holger Schimanski]

Project Role Tab should be extended by adding a new user interface to add and remove users to project roles. This new user interface should be able for specific project member steered by an additional project permission to Edit Project Roles. This would be an alternative way to edit project role membership and you don’t need project-admin permission for it.

The list of project roles should be limited to the roles identified as relevante by Project Role Tab. Could be useful to allow only adding users and not goups to project roles to simplify the functionality.

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