Add Search Curation Settings to ApplicationSettings

What does this MR do and why?

We are rolling out index lifecycle management for search indices. Part of this process is to have configurable settings to have lower thresholds in staging than in production.

For the time being, this will not be accessible in the UI.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.