Marking span in link_to's text as html_safe

Prior to this change the following was rendered as the inner html of
the generated a tag:

```
&lt;span class=&quot;canonical-image&quot;&gt;&lt;/span&gt;
```

With this change the generated text is:

```
<span class="canonical-image"></span>
```

Addresses DLTP-267
Related to DLTP-227
Merge pull request #67 from jeremyf/tidying-up-canonical-image-for-thumbnails

Marking span in link_to's text as html_safe