MM-14686 Send all image proxy requests through /api/v4/image (#10775)

* MM-14686 Implement /api/v4/image when proxy is disabled

* MM-14686 Send all image proxy requests through /api/v4/image

* Update unit tests

services/imageproxy/imageproxy_test.go

@@ -0,0 +1,86 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package imageproxy
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetProxiedImageURL(t *testing.T) {
+	siteURL := "https://mattermost.example.com"
+
+	imageURL := "http://www.mattermost.org/wp-content/uploads/2016/03/logoHorizontal.png"
+	proxiedURL := "https://mattermost.example.com/api/v4/image?url=http%3A%2F%2Fwww.mattermost.org%2Fwp-content%2Fuploads%2F2016%2F03%2FlogoHorizontal.png"
+
+	for _, test := range []struct {
+		Name     string
+		Input    string
+		Expected string
+	}{
+		{
+			Name:     "should proxy an image",
+			Input:    imageURL,
+			Expected: proxiedURL,
+		},
+		{
+			Name:     "should not proxy a relative image",
+			Input:    "/static/logo.png",
+			Expected: "/static/logo.png",
+		},
+		{
+			Name:     "should not proxy an image on the Mattermost server",
+			Input:    "https://mattermost.example.com/static/logo.png",
+			Expected: "https://mattermost.example.com/static/logo.png",
+		},
+		{
+			Name:     "should not proxy an image that has already been proxied",
+			Input:    proxiedURL,
+			Expected: proxiedURL,
+		},
+	} {
+		t.Run(test.Name, func(t *testing.T) {
+			assert.Equal(t, test.Expected, getProxiedImageURL(test.Input, siteURL))
+		})
+	}
+}
+
+func TestGetUnproxiedImageURL(t *testing.T) {
+	siteURL := "https://mattermost.example.com"
+
+	imageURL := "http://www.mattermost.org/wp-content/uploads/2016/03/logoHorizontal.png"
+	proxiedURL := "https://mattermost.example.com/api/v4/image?url=http%3A%2F%2Fwww.mattermost.org%2Fwp-content%2Fuploads%2F2016%2F03%2FlogoHorizontal.png"
+
+	for _, test := range []struct {
+		Name     string
+		Input    string
+		Expected string
+	}{
+		{
+			Name:     "should remove proxy",
+			Input:    proxiedURL,
+			Expected: imageURL,
+		},
+		{
+			Name:     "should not remove proxy from a relative image",
+			Input:    "/static/logo.png",
+			Expected: "/static/logo.png",
+		},
+		{
+			Name:     "should not remove proxy from an image on the Mattermost server",
+			Input:    "https://mattermost.example.com/static/logo.png",
+			Expected: "https://mattermost.example.com/static/logo.png",
+		},
+		{
+			Name:     "should not remove proxy from a non-proxied image",
+			Input:    imageURL,
+			Expected: imageURL,
+		},
+	} {
+		t.Run(test.Name, func(t *testing.T) {
+			assert.Equal(t, test.Expected, getUnproxiedImageURL(test.Input, siteURL))
+		})
+	}
+}