RSS feeds from Crucible review searches do not truncate results

Just noticed that several RSS feeds we have in Crucible (e.g. Custom Review Search) include all matching reviews in the results. This may result in several thousands of reviews to be built, it takes time and resources to build that and potentially degrade overall Crucible performance.
Typical RSS clients reads RSS on a scheduled basis which means on popular instances there might be several feeds being built at any given moment.
Usually RSS are meant to bring just the recent results, we should truncate results to the 10 or couple dozens at most recent results.
Note this feature might be potentially used as export tool to import review data into other data sources, e.g. Excel, see CRUC-6437. Perhaps we want to offer better way of exporting data before we tackle this issue.