Handle creating a nested group on MySQL correctly

This fixes the specs that are currently broken on master concerning Nested Groups.

The Groups::NestedCreateService will now raise an error when trying to import a subgroup when that is not supported.

I've updated the tests to reflect that.

@smcgivern WDYT?

assigned to @smcgivern

@reprazent makes sense to me - thanks

changed milestone to %10.0

approved this merge request

enabled an automatic merge when the pipeline for 529a07bd succeeds

@smcgivern The EE-port passed, but I can't seem to get this one to pass because of https://gitlab.com/gitlab-org/gitlab-ce/issues/34323

canceled the automatic merge

merged

mentioned in commit f92a80b4