Add migrations for import placeholder user limits

What does this MR do and why?

This MR adds four tiers of plan limits to limit the number of import placeholder users that can be created per root namespace during an import, and sets the limit values for GitLab.com plans.

Which of the four tiers of limits that apply will be determined by seat count.

A follow-up MR !162103 (merged) adds the application logic for the limits.

User mapping - Add placeholder user limit (#455903 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #455903 (closed)