Add ability for users to rate issues

[bpepple]

Summary

• Add a 5-star community rating for issues (issue_ratings app), separate from the existing personal collection rating — an HTMX widget on the issue detail page, plus average_rating/rating_count on the issue detail API endpoint.
• Refactor the now-duplicated rating model/view/template logic shared with reading_lists' existing rating feature into comicsdb (AbstractRating, parse_rating_action/apply_rating_update, a shared rating_widget.html partial), and trim average_rating/rating_count back off the high-traffic issue list endpoint, keeping them on detail only.
• Keep a user's personal collection rating (user_collection) and their community issue rating in sync via a signal, and add a backfill_issue_ratings management command to sync pre-existing collection ratings for users upgrading from before this feature.