Create a confirmation modal for the new graph preview

[justinrobots]

1. Add a confirmation modal that describes the new graph preview and allows users to continue to view the graph on private repos for a limited number of days. This will take the place of the existing 3 day preview trial modal for the graph.
2. Use the messaging from the existing modal (see image below), but change Continuing gives you 3 days to preview the Commit Graph and local Pro features. to Continuing gives you 3 days to preview the Commit Graph on private repositories.
3. Add telemetry for when the graph preview starts

Example of existing modal to be replaced: