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]

@petersalomonsen Thank you for your contribution! Your pull request is now a part of the Race of Sloths! Are you going to win race-of-sloths this month? If so, you should speed up! Run, Sloth, run!

[

<source media="(prefers-color-scheme: light)" srcset="https://badge.race-of-sloths.com/petersalomonsen?type=bot&theme=light">
<img alt="Shows profile picture for the author of the PR" src="https://badge.race-of-sloths.com/petersalomonsen?type=bot&theme=white">

](https://race-of-sloths.com/profile/petersalomonsen)

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!

@bucanero, please score the PR with @race-of-sloths score [1/2/3/5/8/13]. The contributor deserves it. If no scoring is provided within 24 hours, this PR will be scored as 1 🦥

[bucanero]

@race-of-sloths score 2