Oops, fix bug with empty subtitles in Item Getting Guide

Oh weird, even with `flush: true`, `content_for` will ignore an empty
block and *not* flush out the previous content. This could cause rows
whose subtitles *should* have been empty (e.g. no NC trade value) to
display the previous row's value instead.

Let's make this whole situation a bit more robust by having the
*template* clear out the subtitle right before calling the block. That
way, a previous row's value *can't* get in, no matter what.

app/views/items/_item_list_row.html.haml

@@ -1,4 +1,6 @@
--# Run the block first, so we can get the subtitle content if provided!
+-# Run the block first, so we can get the subtitle content if provided! Clear
+-# out any old values beforehand, too.
+- content_for :subtitle, "", flush: true
 - content = yield
 - subtitle = yield :subtitle