fix: resolve translated post slugs (D-58)

GET /_journal/api/v1/posts/{slug} now looks the slug up in translate's
slug index for the request locale after the default-locale slug and
before the numeric id. TestTranslatedSlug covers the translated slug,
the default slug, the id and an unknown slug.

controllers/api/posts.go

@@ -128,7 +128,7 @@ func Show(app *backpack.App) http.HandlerFunc {
 		}
 		db = db.WithContext(r.Context())
 		slug := strings.TrimSpace(r.PathValue("slug"))
-		post, err := findPostBySlugOrID(db, slug)
+		post, err := findPostBySlugOrID(db, slug, requestLocale(r))
 		if err != nil || post == nil {
 			writeAPIError(w, http.StatusNotFound, errPostNotFound)
 			return