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/secret/secret_test.go

@@ -13,7 +13,7 @@ import (
 func TestNew(t *testing.T) {
 	result, err := New()
 	assert.NoError(t, err)
-	assert.True(t, len(result) > 32)
+	assert.True(t, len(result) == 44)
 
 	result2, err := New()
 	assert.NoError(t, err)