MM-40450: Follow redirects in image proxy (#20516)

For the local proxy, we weren't following redirects.
We remove the CheckRedirect function and use the default
function, which is good enough for our purposes.

https://mattermost.atlassian.net/browse/MM-40450

```release-note
NONE
```

services/imageproxy/local.go

@@ -64,9 +64,6 @@ func makeLocalBackend(proxy *ImageProxy) *LocalBackend {
 	}
 
 	client := proxy.HTTPService.MakeClient(false)
-	client.CheckRedirect = func(newreq *http.Request, via []*http.Request) error {
-		return http.ErrUseLastResponse
-	}
 
 	return &LocalBackend{
 		proxy:   proxy,

services/imageproxy/local_test.go

@@ -193,6 +193,37 @@ func TestLocalBackend_GetImage(t *testing.T) {
 		_, err = ioutil.ReadAll(resp.Body)
 		require.NoError(t, err)
 	})
+
+	t.Run("Redirect", func(t *testing.T) {
+		var mock *httptest.Server
+		handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			switch r.URL.Path {
+			case "/image.png":
+				w.Header().Set("Location", mock.URL+"/image2.png")
+				w.WriteHeader(http.StatusMovedPermanently)
+			case "/image2.png":
+				w.Header().Set("Cache-Control", "max-age=2592000, private")
+				w.Header().Set("Content-Type", "image/png")
+				w.Header().Set("Content-Length", "10")
+
+				w.WriteHeader(http.StatusOK)
+				w.Write([]byte("1111111111"))
+			}
+		})
+
+		mock = httptest.NewServer(handler)
+		defer mock.Close()
+
+		proxy := makeTestLocalProxy()
+
+		recorder := httptest.NewRecorder()
+		request, _ := http.NewRequest(http.MethodGet, "", nil)
+		proxy.GetImage(recorder, request, mock.URL+"/image.png")
+		resp := recorder.Result()
+
+		require.Equal(t, http.StatusOK, resp.StatusCode)
+		assert.Equal(t, "10", resp.Header.Get("Content-Length"))
+	})
 }
 
 func TestLocalBackend_GetImageDirect(t *testing.T) {