Unify hashing for avatar (#22289)

- Unify the hashing code for repository and user avatars into a
function.
- Use a sane hash function instead of MD5.
- Only require hashing once instead of twice(w.r.t. hashing for user
avatar).
- Improve the comment for the hashing code of why it works.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Yarden Shoham <hrsi88@gmail.com>

modules/avatar/hash.go

@@ -0,0 +1,28 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package avatar
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"strconv"
+)
+
+// HashAvatar will generate a unique string, which ensures that when there's a
+// different unique ID while the data is the same, it will generate a different
+// output. It will generate the output according to:
+// HEX(HASH(uniqueID || - || data))
+// The hash being used is SHA256.
+// The sole purpose of the unique ID is to generate a distinct hash Such that
+// two unique IDs with the same data will have a different hash output.
+// The "-" byte is important to ensure that data cannot be modified such that
+// the first byte is a number, which could lead to a "collision" with the hash
+// of another unique ID.
+func HashAvatar(uniqueID int64, data []byte) string {
+	h := sha256.New()
+	h.Write([]byte(strconv.FormatInt(uniqueID, 10)))
+	h.Write([]byte{'-'})
+	h.Write(data)
+	return hex.EncodeToString(h.Sum(nil))
+}