fix(rss): River entries include their full correct summaries again

This causes the rss viewtype's river/item view to fall back to the
item's default viewtype in order to capture custom logic that
determines what the summaries should be, since the river's summary
view (`river/elements/summary`) doesn't know how to handle all
possible river items, especially now that we have a `target` field
in the river.

Fixes #6901
Merge pull request #7015 from ewinslow/river-rss-b6901

fix(rss): River entries include their full correct summaries again