Allow whitelisting for "external collaborator by default" setting

Gemfile

Gemfile.lock

Gemfile.rails5.lock

...ts/pages/admin/application_settings/account_and_limits.js

app/assets/javascripts/pages/admin/index.js

app/assets/javascripts/pages/admin/users/new/index.js

app/helpers/application_settings_helper.rb

app/helpers/users_helper.rb

app/models/application_setting.rb

app/services/users/build_service.rb

app/validators/js_regex_validator.rb

...s/admin/application_settings/_account_and_limit.html.haml

app/views/admin/users/_access_levels.html.haml

...gs/unreleased/feature-whitelist-new-users-as-internal.yml

...8125206_add_user_internal_regex_to_application_setting.rb

db/schema.rb

doc/user/permissions.md

locale/gitlab.pot

spec/features/admin/admin_settings_spec.rb

spec/features/admin/admin_users_spec.rb