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

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>