Refactor zones

- Rename "zonefile_setting" to "zone"
- Remove version

#475

spec/features/admin/dns/zones/delete_spec.rb

@@ -0,0 +1,16 @@
+require 'rails_helper'
+
+RSpec.feature 'Deleting zone in admin area' do
+  given!(:zone) { create(:zone) }
+
+  background do
+    sign_in_to_admin_area
+  end
+
+  scenario 'deletes zone' do
+    visit edit_admin_zone_url(zone)
+    click_link_or_button t('admin.dns.zones.edit.delete_btn')
+
+    expect(page).to have_text(t('admin.dns.zones.destroy.destroyed'))
+  end
+end