Allow downgrade when no users were added in previous period

What does this MR do?

It compares the restricted users count against the current active users when the customer is downgrading and hasn't added more users in the previous period.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Because customers were unable to upload their license keys due to validation errors.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-com/customers-gitlab-com/issues/103