WIP: Add GitLab seed to populate DB with data necessary to performance test LDAP group sync

A WIP seed/fixture to create data needed to performance test LDAP group sync. We can debate how many group links, etc., but at least this is recording in a WIP MR now and can't be lost again. It's not perfect, but it served the purpose.

I considered where to put this file - in GitLab code base or GDK. I opted to put it here because the fixture didn't make sense in the db/ directory in GitLab with the other seeds. This seed should only be loaded on-demand because it takes a really long time. A developer can easily use load to import the data using the Rails console - or we can wrap it in a nifty command.

cc/ @jacobvosmaer-gitlab

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