Preview: Story Map description should fall back to first chapter text

[DerekCaelin]

Description

When previewing a Terraso Link, description should be user provided or first chapter text. Link preview currently shows old "Landscapes" language.

Steps To Reproduce

1. Take link to story map: https://app.terraso.org/tools/story-maps/09314324/bosque-protector-comunitario-yawa-jee
2. Paste into slack
3. See issue.

Expected behavior

Preview text should be initial characters of first chapter, e.g. "El Bosque y Vegetación Protector Yawa Jee se localiza en el corazón de la Amazonía Ecuatoriana..."

Actual behavior

Preview text is "Terraso is a set of free and powerful software tools to manage your landscape, protect your environment, and promote human livelihoods."