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/user.go

@@ -22,7 +22,6 @@ import (
 	"unicode/utf8"
 
 	"code.gitea.io/gitea/modules/base"
-	"code.gitea.io/gitea/modules/generate"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
@@ -746,7 +745,7 @@ func IsUserExist(uid int64, name string) (bool, error) {
 
 // GetUserSalt returns a random user salt token.
 func GetUserSalt() (string, error) {
-	return generate.GetRandomString(10)
+	return util.RandomString(10)
 }
 
 // NewGhostUser creates and returns a fake user for someone has deleted his/her account.