Implement a Pro feature preview for the graph

[justinrobots]

Implement a new Pro feature preview that more flexibly allows access to specific Pro features for a certain number of days. The current 3 day preview trial will be deprecated.

This Pro feature preview will allow users to access the Commit Graph on private repositories for 3 days without an account.

• Eligibility for this Pro feature preview will be the same as the current 3 day trial
• When an eligible user opens the graph for the first time on a private repository, prompt them with a modal that describes the preview and asks them to continue to start the preview. After continuing, allow the user to view the graph on private repositories for the duration of the preview.
• After the preview expires, prompt the user to start a trial or upgrade to Pro after opening the graph on a private repository again.

Add telemetry that captures when a Pro feature preview starts