add analytics with posthog

[petersalomonsen]

This PR adds analytics with Posthog.

It depends on the environment variables REACT_APP_PUBLIC_POSTHOG_KEY and REACT_APP_PUBLIC_POSTHOG_HOST being set.

This is how it looks like inside the POSTHOG web analytics

[petersalomonsen]

@race-of-sloths include

[race-of-sloths]

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Current status: executed

Reviewer	Score
@bucanero	2

Your contribution is much appreciated with a final score of 2!
You have received 48 (20 base + 25 weekly bonus + 5% lifetime bonus) Sloth points for this contribution

@petersalomonsen, welcome to the Hall of Sloth fame! You have been honored with a Bronze rank and a 5% lifetime bonus to all rating points you earn with your next contributions!

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[race-of-sloths]

🥁 Score it!

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[bucanero]

@race-of-sloths score 2