Add page dashboard

This MR introduces dashboards for individual pages and a widget with links to these dashboards, fulfilling the requirements of T337215 and T337214, and finalizing the MVP.

This new feature required a significant refactor, so this MR is quite big. The most important technical change is the introduction of cache-id, which decouples identification of cached records from individual page piles. Previously, pagepile-id served as cache-id. This is still the case for the collection dashboard (which displays data for the entire page pile). For individual pages, the cache-id uniquely identifies cached data for a given page on a given wiki, making it possible to reuse more data without querying the APIs or databases.

Bug: T337215