Use single shared random string generation function (#15741)

* Use single shared random string generation function

- Replace 3 functions that do the same with 1 shared one
- Use crypto/rand over math/rand for a stronger RNG
- Output only alphanumerical for URL compatibilty

Fixes: #15536

* use const string method

* Update modules/avatar/avatar.go

Co-authored-by: a1012112796 <1012112796@qq.com>

Co-authored-by: a1012112796 <1012112796@qq.com>

modules/avatar/avatar.go

@@ -12,10 +12,9 @@ import (
 
 	// Enable PNG support:
 	_ "image/png"
-	"math/rand"
-	"time"
 
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/util"
 
 	"github.com/issue9/identicon"
 	"github.com/nfnt/resize"
@@ -29,8 +28,11 @@ const AvatarSize = 290
 // in custom size (height and width).
 func RandomImageSize(size int, data []byte) (image.Image, error) {
 	randExtent := len(palette.WebSafe) - 32
-	rand.Seed(time.Now().UnixNano())
-	colorIndex := rand.Intn(randExtent)
+	integer, err := util.RandomInt(int64(randExtent))
+	if err != nil {
+		return nil, fmt.Errorf("util.RandomInt: %v", err)
+	}
+	colorIndex := int(integer)
 	backColorIndex := colorIndex - 1
 	if backColorIndex < 0 {
 		backColorIndex = randExtent - 1