Admin feature: Better table for the items list of BackgroundActivity

[prioux]

Right now, an admin can inspect a BAC and they get a table with the list of items. The items are colorized: black for not processed yet, green for processed successfully, red for failures.

Each BAC subclass should provide a hint to the view system about the type of items they manage, so that the view code could make this tableeven more useful:

1. if the items are IDs to a model, show the link to the object instead
2. add a prefix ✅ and ❌ in front of the items to represent success or failure instead of coloring them
3. have a view option (selected by a query param) to view the raw values vs the pretty values (e.g. plain IDs like right now, vs links)
4. if the BAC is a producer of new IDs (in its messages array), make thos IDs into links to, within the table (second row in the cell?)
5. if the BAC is a producer of messages, have the messages also be available within each cell (assuming we can make the message short enough) or as a legend at the bottom of the table. A lot of standard exceptions are already summarized by the BAC framework itself (e.g. ActiveRecord::RecordNotFound exception are simply recorded as 'NotFound')