Trials should be its own plan

Summary

Currently, when a trial is taken, the GitLab.com namespace is set to the Ultimate plan and receive the entitlements (features, CI minutes, storage limits) of the Ultimate paid subscription. This process limits our ability to easily recognize and manage trials apart from paid subscriptions.

Proposal

Create a new plan for Premium and Ultimate Trials. When a trial is accepted, utilize this plan instead of the paid Premium/Ultimate plan.