Redirect Old URL to New URL for Renamed Recipes

Recipe URLs are based on the slug generated from the recipe name. When the name changes, so does the slug. This breaks all external links to the old version of the recipe. We should probably keep a record of aliases for a recipe and either return the appropriate recipe under that alias' URL, or return an HTTP redirect (301) to the new URL.

No files or tests are named. Start by locating the recipe URL and slug-generation entry points, then trace how a renamed recipe is looked up. Done means old recipe URLs resolve reliably, either by serving the recipe through an alias or returning a 301 redirect to its new URL, with coverage for renamed recipes.

Written by the indexing model from the issue text.