Refactor `feed-checker.js` to use `feed-page-checker.js`

[nickevansuk]

Ensure that feed-checker.js uses feed-page-checker.js, and that it therefore runs this line of logic for all rules:

rpde-validator/src/feed-page-checker.js

Line 37 in 0229cd7

const isItemDuplicationPermissible = isInitialHarvestComplete || cacheControlComponents.public;

This will ensure that the Cache Control errors are not displayed in the Validator GUI for feeds where a cache is in use.