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>

tag.hbs

@@ -10,6 +10,7 @@
           {{description}}
         {{else}}
           {{plural ../pagination.total empty='posts' singular='% post' plural='% posts'}}
+              • <a  class="mind-post-header_tag" href="/tags/">see all tags</a>
         {{/if}}
       </p>
     </div>