Try absolute path for Propshaft assets

[aduth]

🛠 Summary of changes

Changes url(...) to reference assets using absolute (slash-prefixed) paths, in hopes of fixing issues observed in deployed environments, and following related migration guidance:

[...]Propshaft assumes all paths are relative to the path of the file it’s processing. Since it was processing a css file inside the theme folder, it will also look for hero.jpg in the same folder.

By adding a / at the start of the path we are telling Propshaft to consider this path as an absolute path.

https://github.com/rails/propshaft/blob/main/UPGRADING.md#3-migrate-from-sprockets-to-propshaft

📜 Testing Plan

1. Go to http://localhost:3000
2. Observe ⬇️ caret image next to "Here's how you know in banner"

[aduth]

Works locally and in my personal sandbox, 🤞 for other deployed environments.