Fix: #870

This might be a reincarnation of #169

To reproduce:

1. Open https://playground.wordpress.net/?plugin=web-stories&php-extension-bundle=kitchen-sink
2. Go to Dashboard -> Stories
3. Hover over a template and click on "Use template"
4. This would normally trigger a REST API POST request with a (relatively large) body
5. See memory access out of bounds error in console

Are there some body size limits with playground that one should be aware of?