8.5
billing: Update copy, modal for self-hosted downgrades and trials.
remote_billing: Add endpoint and a helper to make deactivation links.
billing: Make various buttons on billing page work.
sponsorship: Show different discount text for self hosted orgs.
upgrade: Provide billing_base_url in page_params.
billing: Offer release announcement subscriptions.
stripe: Render upgrade page for remote servers on legacy plan.