feat(17-02): tree reorder page and singleton form in the admin

- ReorderView: indented tree with drag handle, move up/down, outdent and
  indent (buttons and arrow keys on the handle), pointer drag before, after
  or inside a row and never into its own subtree; one POST with the whole
  node set 400 ms after the last move, flushed on leave; a failed save
  restores the saved tree, a 409 reloads; loading, empty, load-failure and
  403 states; partition selector
- ListView: outline Reorder button before New record, passing the filtered
  partition
- SettingsIndexView opens a singleton controller's record through
  GET …/singleton; FormView in singleton mode has no Delete and returns to
  the settings index
- reorder.* and messages.list.reorder phrase keys (en, pl), smoke spec, docs

docs/backend/admin-controllers.md

@@ -455,6 +455,8 @@ A controller that edits exactly one record, such as a site-wide definition, impl
 - The controller needs a `config_form.yaml` and no `config_list.yaml`. Its list, create, bulk and delete routes answer 404 `not_found`, and the singleton route answers 404 on any other controller.
 - A settings entry whose `Controller` names the singleton controller links to it; see [Settings](settings.md).
 
+In the admin, clicking such a settings entry asks the singleton route for the id and opens the record's form directly. The form has no Delete button, and its back button, Cancel and Save and close return to the settings index. A settings link to a controller that is not a singleton gets a 404 from that route and opens the controller's list as before.
+
 ## Record actions
 
 A record action runs on one record, as a button on its screen: activating an account, lifting a ban. The controller registers its record actions through `pact.HasAdminRecordActions` (`PeopleController` above registers two), and `recordActions` in `config_form.yaml` lists the ones the form offers, in display order: