Backfill options and settings

There's a custom store that watches for changes to the homepage and fetches previews and backfills for live previews.

The frontend and backend editorial preview use the same PHP code to determine the backfill data.

Backfill logic

On the homepage, any Nova Card slot not manually curated or pinned will backill according to the selected/configured filters.

Content that is backfilled will also be deduplicated. This logic is applied across all modules.

Paginated content on the "blogroll" (i.e. /page/2) is not deduplicated.

Post types - Select one or more.
Categories - Filter by a single category.
Tags - Filter by a single tag.
Trending - Sort by "Most Viewed" or "Most Commented" within a selected timeframe (various options available). Uses the Top Posts (opens in a new tab) in pmc-utils, which uses Jetpack stats.