fix: fallback to find images in content

[yin1999]

Summary

Fixes #5652.

Solution

To fallback to serve images which is not in translated-content but in content:

1. find all images which in content repo but not in translated-content repo (only match names), and write to fallback-paths.json
2. in aws lambda request handler, check if the requested file is only in content repo. If so, then rewrite the request uri to /en-us/.

How did you test this change?

I've run the build.js locally, there are about 18000 fallback paths now.