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>

models/token.go

@@ -10,8 +10,8 @@ import (
 	"time"
 
 	"code.gitea.io/gitea/modules/base"
-	"code.gitea.io/gitea/modules/generate"
 	"code.gitea.io/gitea/modules/timeutil"
+	"code.gitea.io/gitea/modules/util"
 
 	gouuid "github.com/google/uuid"
 )
@@ -40,7 +40,7 @@ func (t *AccessToken) AfterLoad() {
 
 // NewAccessToken creates new access token.
 func NewAccessToken(t *AccessToken) error {
-	salt, err := generate.GetRandomString(10)
+	salt, err := util.RandomString(10)
 	if err != nil {
 		return err
 	}