Add <symbol> for a selected seat

When rendering the list of `Seat` records for a `Floor`, render [`<use>`
elements][use] referring to the new `#seat-icon-selected` version of a
seat based on whether or not they're in the visitor's `Cart`.

Desktop
---

![selected seat icon](https://user-images.githubusercontent.com/2575027/77386360-0288da80-6d61-11ea-8d80-fe46c3f53706.png)

[use]: https://developer.mozilla.org/en-US/docs/Web/SVG/Element/use

app/views/seats/index.html.erb

@@ -29,6 +29,15 @@
             <circle fill="#37b24d" r="12" cx="12" cy="12"></circle>
             <circle fill="#ffffff" r="6" cx="12" cy="12"></circle>
           </symbol>
+
+          <symbol
+            id="seat-icon-selected"
+            width="24"
+            height="24"
+            viewbox="0 0 24 24"
+          >
+            <circle fill="#37b24d" r="12" cx="12" cy="12"></circle>
+          </symbol>
         </svg>
 
         <% sections.each do |section| %>
@@ -39,7 +48,11 @@
                 aria-label="<%= seat.row_number %>"
               >
                 <use
-                  xlink:href="#seat-icon-unselected"
+                  <% if Current.cart.include?(seat) %>
+                    xlink:href="#seat-icon-selected"
+                  <% else %>
+                    xlink:href="#seat-icon-unselected"
+                  <% end %>
                   width="12px"
                   height="12px"
                   x="<%= seat.x %>"