feat(web-analytics): Add staleness check and update warning

[robbie-c]

Problem

https://posthog.slack.com/archives/C03JJKLEC8L/p1700061413856629?thread_ts=1700056233.185239&cid=C03JJKLEC8L

Changes

• Don't just check that the $pageleave/$pageview events have ever been sent, check that they have also been sent recently (I just used the staleness constant we already had, but might want to shorten this if it turns out to be too long).
• Change the error messages to include more relevant docs.

New error screenshots (ignore the fact that there obviously are pageviews)

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How did you test this code?

Ran it manually

[github-actions]

Size Change: 0 B

Total Size: 2.01 MB

ℹ️ View Unchanged

Filename	Size
frontend/dist/toolbar.js	2.01 MB

_{compressed-size-action}