support set gravatar as user avatar provider

pkg/utils/avatar.go

@@ -0,0 +1,11 @@
+package utils
+
+import "fmt"
+
+const gravatarUrlFormat = "https://www.gravatar.com/avatar/%s"
+
+// GetGravatarUrl returns the Gravatar url according to the specified user email address
+func GetGravatarUrl(email string) string {
+	emailMd5 := MD5EncodeToString([]byte(email))
+	return fmt.Sprintf(gravatarUrlFormat, emailMd5)
+}