Change the semantic of adding / removing users to a group

[alexander-yevsyukov]

Now commands say JoinGroup and LeaveGroup. Although it's all liberal and nice, it does not work this way in general. Some groups may be open for joining, some not, and require moderators or admins to add or remove users.

For a relaxed case we can always allow users to issue a command for removing himself from a group. Allow or not would be the decision of an application.