Notify GitLab.com users when their plan might change/has changed

Problem

Once a subscription is downgraded/upgraded, we don't currently let users know that it's happened.

Proposal

• Implement an in-app banner notification to let users know that their instance will be downgraded soon and they should contact their billing administrator to avoid a service interruption. This could also potentially help to prevent churn.
• Implement an in-app banner notification that lets users know that their instance has been downgraded/upgraded to X plan.
• Enable email notifications for the above for the billing administrators.

Result

Users are better informed of what features are available to them at any given time. In the event of an upgrade, this is an opportunity for the Product Marketing team to help users learn about their newly available features.

cc @williamchia @jameslopez @rdavila