FEATURE: Use responsive image sizes in post stream (PR #13343)

Followup to DEV: Experiment with relative image sizes and native image lazy loading by pmusaraj · Pull Request #12759 · discourse/discourse · GitHub

We have been testing this change (see original PR description) on several Discourse instances for a few weeks/months and it seems to be holding up well.

(In draft state since I will be away until June 16.)




Love to delete code!