Allow users to downgrade their plans

A user should be able to self-serve downgrading a paid self-managed license or a GitLab.com plan to a lower tier of service or a lower seat count.

• Downgrade an existing license to a lower tier.
• Downgrade the number of seats on a license (self-managed).
• Factor in any relevant true-up costs.
• Reissue the new license key automatically and deprecate the old one.

Note: If we decide to do this, we should only have the changes apply to the next subscription, not the current one.