Report conversions from Playground

[DylanTinianov]

Instructions

Report to Mixpanel when a project exported from playground is opened in vscode for the first time by that vscode user.

• Maintain a list of exported playground projects in the user's vscode settings where we store the user id.

Playground will export the following:

Also report when an exported project has an initial deployment on emulator. In order to accomplish this, we might need to report this on the emulator / flow-cli side instead of VSCode (especially if users are using flow dev instead of vscode commands).

[sideninja]

Let's make sure there's an opt-out option on the playground export dialog. Also, make sure to explain what we track. We shouldn't do this without explaining it to the user. cc @fotescodev @bthaile