remove anybody from the thread

Issue Summary

Hi

I adding this feature request to version 2.x (1.0 is round the corner)
The message owner (and admin) could have permission to remove anybody from the thread.
(it could use similar function as it is in submodule for adding new users or remove yourself )

you know, somebody could be added by mistake and then you have no option how to remove it from the thread.

thanks
Igor