Only show "Mirror repository" option in New project form when the mirror feature is available to the selected namespace
See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2756#note_38446442.
This could use a similar approach to option 1 in https://gitlab.com/gitlab-org/gitlab-ce/issues/31273#note_37391887, where the value will go into a data attribute instead of being requested over the API.