Add Allow-/Block-List for Migrate & Mirrors (#13610)

* add black list and white list support for migrating repositories

* fix fmt

* fix lint

* fix vendor

* fix modules.txt

* clean diff

* specify log message

* use blocklist/allowlist

* allways use lowercase to match url

* Apply allow/block

* Settings: use existing "migrations" section

* convert domains lower case

* dont store unused value

* Block private addresses for migration by default

* fix lint

* use proposed-upstream func to detect private IP addr

* a nit

* add own error for blocked migration, add tests, imprufe api

* fix test

* fix-if-localhost-is-ipv4

* rename error & error message

* rename setting options

* Apply suggestions from code review

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

docs/content/doc/advanced/config-cheat-sheet.zh-cn.md

@@ -313,6 +313,9 @@ IS_INPUT_FILE = false
 
 - `MAX_ATTEMPTS`: **3**: 在迁移过程中的 http/https 请求重试次数。
 - `RETRY_BACKOFF`: **3**: 等待下一次重试的时间，单位秒。
+- `ALLOWED_DOMAINS`: **\<empty\>**: 迁移仓库的域名白名单，默认为空，表示允许从任意域名迁移仓库，多个域名用逗号分隔。
+- `BLOCKED_DOMAINS`: **\<empty\>**: 迁移仓库的域名黑名单，默认为空，多个域名用逗号分隔。如果 `ALLOWED_DOMAINS` 不为空，此选项将会被忽略。
+- `ALLOW_LOCALNETWORKS`: **false**: Allow private addresses defined by RFC 1918
 
 ## LFS (`lfs`)