Add PlayFab

[lazerwalker]

Initially, adding PlayFab will just get us basic analytics.
Eventually, it can serve the base for features like leaderboards and letting people rate games.

To think about:

• Whether we capture enough data to necessitate a privacy policy. Obviously our goal isn't to sell data, or even collect data worth selling
• A workflow where the title ID is injected at build-time, and the game runs without PlayFab in the local editor and PR preview builds (partly to avoid muddying up data, partly to avoid bots from scraping the repo for the title ID, even though that's technically public info)
• I assume PlayFab dashboard acces can just be given to mods, but there's stuff to think about there