Preventing system users being removed from system groups

additional changes to PR https://github.com/openmicroscopy/openmicroscopy/pull/565
preventing from changing user and group name of system entities
adding getAdministrators helper to the gateway
fixing prohibited action of user/group removal
replacing hardcoded value '0' by getAdminService().getSecurityRoles().systemGroupId
replacing hardcoded group and user ids by values provided via api::IAdmin::getSecurityRoles
removing trailing comma
uncheking admin checkbox when removed from system group
preventing from changing the name of current group in the EventContext, close #11693
fixing issue with passing inexistent object to the template
Merge pull request #1733 from aleksandra-tarkowska/trac-11465-user-group-change-restrictions

Preventing system users being removed from system groups