Cache rendered Markdown fields in the database

## What does this MR do?

Introduces cache fields for Markdown-containing fields in the database, and populates them.

## Why was this MR needed?

Rendering Markdown into HTML is performance-intensive. A Redis cache already exists, but this approach is expected to be more performant and reduce unnecessary cache invalidations.

## What are the relevant issue numbers?

Closes #18337

See merge request !6095