Exploration/billing integration (#6393)

* Add toggle_display_name to Feature; rename Funnels to include user journeys

* Gate exploration availability on Funnels billing feature

* Remove exploration_wildcard_disabled feature flag

* Show upgrade notice for exploration when billing unavailable

* Update exploration availability tests to use billing

* Add preview mocks for the Exploration, Funnels and Properties reports

- Preview mocks are non-interactive, blurred previews of the reports that are rendered behind the CTA when the feature isn't available on the current plan. They give users an at-a-glance idea of what the report looks like without needing to navigate elsewhere. Always heavily blurred in their parent container, so the goal is to be suggestive rather than pixel-perfect.

* Update copy

* Update link to user journeys docs

* Update border color of preview mocks

* unformat

---------

Co-authored-by: Sanne de Vries <sannedv@protonmail.com>