Add Importer User for imports user mapping

What does this MR do and why?

This MR adds Import user to a top level group with a new user type import_user. A new Gitlab::Import::ImportUserCreator is added to provision import_user to a root group so that it can be used when doing user mapping when we can't assign imported data to placeholder users.

It:

1. Creates import user on a root group level for a portable provided (root group/subgroup/project)
2. Returns existing import user if it's present
3. Raises if provided portable doesn't have a root group
4. Raises if user has no permission to admin_group root group
5. Raises if import user failed to be persisted

Created import user doesn't appear in the list of billable users.

Mentions #443553 (closed)

[4] pry(main)> Gitlab::Import::ImportUserCreator.new(current_user: User.first, portable: Project.last).execute

=> #<User id:70 @import_user_group_9_dd85106ade8f8cdffb6dd93d0bb35e26>

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Before	After

How to set up and validate locally

1. Open rails console
2. Run Gitlab::Import::ImportUserCreator.new(User.first, Group.find_by_full_path('mygroup')).execute with your user and group
3. Observe import user is created