Finish creating /tags/ page

The tags page was finally completed, and shows every tag
with its own image.

A "see all tags" link was added in the header of every
(single) tag's page, in case of a missing description.

Also, the "secondary navigation" div now has a relative
position and a higher z-index, so that the tags page's
header does not cast a shadow on it.

Signed-off-by: Lorenzo Dellacà <lorenzo.dellaca@mind-overflow.net>

partials/tag-card-big.hbs

@@ -1,17 +1,8 @@
 <article class="{{post_class}}">
-  <a class="mind-post-card-big_main" href="{{url}}">
-    <img class="mind-post-card-big_feature-image" src="{{img_url feature_image size="m"}}" />
-    <header class="mind-post-card-big_header">
-      <p class="mind-post-card-big_meta"><span class="mind-post-card-big_tag">{{primary_tag.name}}</span>
-        • <time class="post-date" datetime="{{date format='YYYY-MM-DD'}}">{{date format="DD MMMM YYYY"}}</time></p>
-      <h3>{{title}}</h3>
+  <a class="mind-tag-card_main" href="{{url}}">
+    <img class="mind-tag-card_feature-image" src="{{img_url feature_image size="s"}}" />
+    <header class="mind-tag-card_header">
+      <h4>{{name}}</h4>
     </header>
-    <section class="mind-post-card-big_excerpt">
-      <p>{{excerpt words="26"}}</p>
-    </section>
-    <footer class="mind-post-card-big_footer">
-      {{primary_author.name}}
-      {{#if primary_author.profile_image}}<img class="mind-post-card-big_author-image" src="{{img_url primary_author.profile_image size="xxs"}}" alt="Author image" />{{/if}}
-    </footer>
   </a>
 </article>