site: Improve definition of preview path, fix double slash on root

[michaeltaranto]

Centralising the definition of the preview path for deployments to a single env var. This makes it easier to update the path once centrally (e.g. the custom grad-connection site we maintain).

Also fixing a double slash on the production site link when on the root page.

[changeset-bot]

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Latest commit: 41805d2

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