feat: point opensearch.xml at cloud function

[LeoMcA]

Part of #1079

mdn/dex#391 must land first

Wires up the fred side of the quicksearch in OpenSearch:

• removes fred's static opensearch.xml to point at the locale-dependent cloud function one
• fixes a type issue in the quicksearch code
• fixes the first (0-indexed) item of the quicksearch never being returned
• routes the paths added in the above PR to the cloud function in the dev server, necessary for local testing

[github-actions]

f44f9a6 was deployed to: https://fred-pr1664.review.mdn.allizom.net/