insights: store structs use singular `ID` variables for arrays

Created by: leonore

In the backend we have a few structs for store query arguments that use singular ID naming for variables that actually are arrays. It seems this is also only something that happens in insights: https://sourcegraph.com/search?q=context:global+repo:%5Egithub%5C.com/sourcegraph/sourcegraph%24+ID%5Cs%2B%5C%5B%5C%5D&patternType=regexp.

The goal of this ticket is just to remove this tech debt by updating the ID variables to be plural.