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/generate/generate_test.go

@@ -1,24 +0,0 @@
-// Copyright 2020 The Gitea Authors. All rights reserved.
-// Use of this source code is governed by a MIT-style
-// license that can be found in the LICENSE file.
-
-package generate
-
-import (
-	"os"
-	"testing"
-
-	"github.com/stretchr/testify/assert"
-)
-
-func TestMain(m *testing.M) {
-	retVal := m.Run()
-
-	os.Exit(retVal)
-}
-
-func TestGetRandomString(t *testing.T) {
-	randomString, err := GetRandomString(4)
-	assert.NoError(t, err)
-	assert.Len(t, randomString, 4)
-}