[amp story shopping] Remove attachment templates when far from active page

[processprocess]

Description

Attachment templates can potentially build a lot of dom that may not be necessary if the user navigates far away from them. They should be removed from the dom at a certain distance and can be re-built if the user goes back.

Alternatives Considered

Leave them in the dom. It's okay to leave them but it keeps lots of un-needed dom in the document, which may slow down the experience.

Additional Context

Distance can be read using a pattern used in panning-media

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