MM-11272 Add OpenGraph and image dimension metadata to posts (#9313)

* Move OpenGraph code into its own file

* Move OpenGraph image proxying to app layer

* Move test file code out of api4 package

* MM-11272 Add OpenGraph and image dimension metadata to posts

utils/testutils/testutils.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package testutils
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"path/filepath"
+
+	"github.com/mattermost/mattermost-server/utils"
+)
+
+func ReadTestFile(name string) ([]byte, error) {
+	path, _ := utils.FindDir("tests")
+	file, err := os.Open(filepath.Join(path, name))
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	data := &bytes.Buffer{}
+	if _, err := io.Copy(data, file); err != nil {
+		return nil, err
+	} else {
+		return data.Bytes(), nil
+	}
+}