Fix bug where some pages where not shown as tiles.

This commit refactors the `tiles.html` include file so that when the
`site.tiles-source` is pages, pages with `show_tile: false` set in their
Front Matter are not counted as having been shown. Prior to this commit,
such pages were iterated over in the `for` loop, consuming a loop
iteration from the `site.tiles-count` counter.

Put another way, say you have set `tiles-count` to 6, and you have a
total of 10 pages. Of these ten pages, six have `show_tile` set to
`false`. The natural expectation would be that the home page layout
would display four tiles (10 - 6 = 4), but in fact, the exact number of
page tiles shown would depend on the specific lexicographical ordering
of the `site.html_pages` array, which could result in fewer than four
page tiles being shown.

This commit fixes the issue by excluding pages with `show_tile` set to
`false` first, and only then iterating over the remaining pages.

_includes/tiles.html

@@ -1,28 +1,18 @@
+{% if 'posts' == site.tiles-source %}
+    {% assign tiles = site.posts %}
+{% elsif 'pages' == site.tiles-source %}
+    {% assign tiles = site.html_pages | where_exp: "item", "item.show_tile != false" %}
+{% endif %}
 <section id="one" class="tiles">
-  {% for post in site.posts limit:site.tiles-count %}
+  {% for tile in tiles limit:site.tiles-count %}
-  {% if site.tiles-source == 'posts' %}
   <article>
     <span class="image">
-      <img src="{{ post.image }}" alt="" />
+      <img src="{{ tile.image }}" alt="" />
     </span>
     <header class="major">
-      <h3><a href="{{ post.url  | relative_url }}" class="link">{{ post.title }}</a></h3>
+      <h3><a href="{{ tile.url  | relative_url }}" class="link">{{ tile.title }}</a></h3>
       <p>{{ post.description }}</p>
     </header>
   </article>
-  {% endif %}
-  {% endfor %}
-  {% for page in site.html_pages limit:site.tiles-count %}
-  {% if site.tiles-source == 'pages' and page.show_tile != false %}
-  <article>
-    <span class="image">
-      <img src="{{ page.image }}" alt="" />
-    </span>
-    <header class="major">
-      <h3><a href="{{ page.url | relative_url  }}" class="link">{{ page.title }}</a></h3>
-      <p>{{ page.description }}</p>
-    </header>
-  </article>
-  {% endif %}
   {% endfor %}
 </section>