Send one channel its own list of specifications

Until now an override could change a product's words, its prices and its images on one connection, and its specifications went out the same everywhere.

Open a product, choose Overrides, pick the connection, and the Specifications row now lets you choose which of the product's values that channel receives, in the order you want. An empty list publishes none of them. The specifications you choose reach Shopify, BigCommerce and Google; Square and Amazon don't receive them.