Use breadcrumbs logic for tab title

[Twixes]

Is your feature request related to a problem?

Currently we have setPageTitle()s sprinkled around the codebase, which provides users with meaningful PostHog tab titles in the browser tab. However, this way there are many sources of truth for the title and they aren't entirely accurate. For instance, the Insights page is titled "Saved insights", while a feature flag's page just says "Feature flag" when it would be very helpful to show the feature flag's actual key.

Describe the solution you'd like

This exact challange was part of the implementation of breadcrumbs and now it's (mostly) solved by breadcrumbsLogic.
Since the tab title is basically abbreviated breadcrumbs, we should just consolidate all the setPageTitle()s into a single callsite in breadcrumbsLogic for a single and accurate source of truth.

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[Twixes]

This is implemented now.