Fix user avatar name (#8547)

Migrate avatar names to include user ID and the md5 hash.

models/user.go

@@ -521,7 +521,11 @@ func (u *User) UploadAvatar(data []byte) error {
 	}
 
 	u.UseCustomAvatar = true
-	u.Avatar = fmt.Sprintf("%x", md5.Sum(data))
+	// Different users can upload same image as avatar
+	// If we prefix it with u.ID, it will be separated
+	// Otherwise, if any of the users delete his avatar
+	// Other users will lose their avatars too.
+	u.Avatar = fmt.Sprintf("%x", md5.Sum([]byte(fmt.Sprintf("%d-%x", u.ID, md5.Sum(data)))))
 	if err = updateUser(sess, u); err != nil {
 		return fmt.Errorf("updateUser: %v", err)
 	}