refactor: use a better construct than doc.Find(…).First()

As mentioned in goquery's documentation (https://pkg.go.dev/github.com/PuerkitoBio/goquery#Single):

> By default, Selection.Find and other functions that accept a selector string
to select nodes will use all matches corresponding to that selector. By using
the Matcher returned by Single, at most the first match will be selected.
>
> The one using Single is optimized to be potentially much faster on large documents.

internal/mediaproxy/rewriter.go

@@ -87,7 +87,7 @@ func genericProxyRewriter(router *mux.Router, proxifyFunction urlProxyRewriter,
 		}
 	}
 
-	output, err := doc.Find("body").First().Html()
+	output, err := doc.FindMatcher(goquery.Single("body")).Html()
 	if err != nil {
 		return htmlDocument
 	}