Admission-based registrations mode (#10250)

app/controllers/accounts_controller.rb

app/controllers/admin/accounts_controller.rb

app/controllers/admin/dashboard_controller.rb

app/controllers/admin/settings_controller.rb

app/controllers/api/base_controller.rb

app/controllers/api/v1/accounts_controller.rb

app/controllers/auth/registrations_controller.rb

app/controllers/concerns/account_controller_concern.rb

app/helpers/admin/filter_helper.rb

app/helpers/application_helper.rb

app/helpers/home_helper.rb

app/javascript/styles/mastodon/admin.scss

app/mailers/admin_mailer.rb

app/models/account.rb

app/models/account_filter.rb

app/models/form/admin_settings.rb

app/models/user.rb

app/policies/user_policy.rb

app/presenters/instance_presenter.rb

app/serializers/rest/instance_serializer.rb