Add icon-lock class when toggling privacy settings

The design uses icons applied with CSS classes rather than toggling the visibility of a separate lock element.

Fixes #193 
This is minor, but I'm raising it because it's something I've been unable to form a clear opinion about and I'd value the discussion. I flip back and forth between preferring your implementation and the more verbose:
```
{% if not category.is_public %}
class="col-xs-8 icon-lock"
{% else %}
class="col-xs-8"
{% endif %}"
```