🧹 Furniture: use FurniturePlacement#location in forms (#1180)

🧹 `Furniture`: use `location` rather when building new Furniture

- Extracted from: #1083
- #74
- #709

app/views/furniture_placements/_new.html.erb

@@ -1,10 +1,10 @@
 <fieldset id="new_furniture_placement">
   <h3><%= t('rooms.place_furniture_heading') %></h3>
-  <%= form_with model: [furniture_placement.room.space, furniture_placement.room, furniture_placement] do | placement_form | %>
+  <%= form_with model: furniture_placement.location do | placement_form | %>
     <%= placement_form.hidden_field :slot, value: furniture_placement.room.furniture_placements.count %>
     <%= placement_form.select :furniture_kind, Furniture::REGISTRY.keys.map { |k| [k.to_s.titleize, k] } %>
     <footer>
       <%= placement_form.submit %>
     </footer>
   <%- end %>
-</fieldset>
+</fieldset>