Oops, fix recent bug in Item Getting Guide for most item types

Oh oops, I added an optional `subtitle` argument for the item table
list row template, but didn't realize that it would crash in cases when
not provided! Now, we add an initializer to set it to `nil` if
undefined.

app/views/items/_item_list_row.html.haml

@@ -1,3 +1,5 @@
+- subtitle = nil unless defined? subtitle
+
 %tr
 	%td.thumbnail-cell
 		= link_to item_thumbnail_for(item), item, target: "_blank",