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>

services/user/user.go

@@ -5,7 +5,6 @@ package user
 
 import (
 	"context"
-	"crypto/md5"
 	"fmt"
 	"image/png"
 	"io"
@@ -241,11 +240,7 @@ func UploadAvatar(u *user_model.User, data []byte) error {
 	defer committer.Close()
 
 	u.UseCustomAvatar = true
-	// 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)))))
+	u.Avatar = avatar.HashAvatar(u.ID, data)
 	if err = user_model.UpdateUserCols(ctx, u, "use_custom_avatar", "avatar"); err != nil {
 		return fmt.Errorf("updateUser: %w", err)
 	}