Add API endpoint to look up entities directly based on some non-BB identifier; similar to non‐MBID lookups.

This could return a list of possible matches (where one external id matches several BBIDs) – maybe possibly a HTTP redirect if only a single match exists.

This should probably live under /external-identifier/<:ID-name>/<:ID> (e.g., /external-identifier/musicbrainz/a9d75482-6c6e-49a3-87db-b228373923dd, /external-identifier/wikidata/Q1413795) or similar to not "pollute" the main root namespace as more identifiers are added.