perf(mediaproxy): convert to []byte in ProxifyAbsoluteURL only once

There is no need to call `[]byte(mediaURL)` twice. It might make a minor
difference performance-wise on images-heavy feed items with long media URL,
like autogenerated CDN ones.

internal/mediaproxy/url.go

@@ -24,10 +24,12 @@ func ProxifyRelativeURL(router *mux.Router, mediaURL string) string {
 		return proxifyURLWithCustomProxy(mediaURL, customProxyURL)
 	}
 
+	mediaURLBytes := []byte(mediaURL)
+
 	mac := hmac.New(sha256.New, config.Opts.MediaProxyPrivateKey())
-	mac.Write([]byte(mediaURL))
+	mac.Write(mediaURLBytes)
 	digest := mac.Sum(nil)
-	return route.Path(router, "proxy", "encodedDigest", base64.URLEncoding.EncodeToString(digest), "encodedURL", base64.URLEncoding.EncodeToString([]byte(mediaURL)))
+	return route.Path(router, "proxy", "encodedDigest", base64.URLEncoding.EncodeToString(digest), "encodedURL", base64.URLEncoding.EncodeToString(mediaURLBytes))
 }
 
 func ProxifyAbsoluteURL(router *mux.Router, mediaURL string) string {