[template_group_leave] handles the logic needed to exit a user from a group.
[functions_groups] Added a RemoveUserFromGroup() method to allow users to exit a group.
[mxpress_index] Added include for page with title 'Leave Group'.
[single-kzm_group] Added link to allow members of a group to exit/leave the group.
[functions_groups] Added GetGroupMemberState() method to get the current user's group member state. Renamed removeUserFromGroup() method to SetMemberStateInGroup(), also refactored it to allow the state to be set according to given parameter.
[template_group_leave] Use SetMemberStateInGroup() method to deactivate a user froma  group.