[staging] Navigating to frontend route directly gives 404

[glencbz]

Steps to reproduce:

• Open a browser, open the console
• Enter https://app2-staging.callhome.sg/support-detail into the address bar and navigate to it
• You should see a 404 error in the console (the page may render if the assets were cached)

This happens because our current urlmap resolves the paths directly to files on GCS e.g. /support-detail resolves to the nonexistent /support-detail file on GCS. To fix this, we need to redirect these other paths to /index.html.

We need to be careful though, because there are a bunch of other paths that we do want to resolve to GCS files:

The best approach would probably be to allowlist just the paths we care about (it should just be the ones here), and route everything else to /index.html by default. That would also give us a chance to do 404 handling if the user decides to navigate to /does-not-exist.