Fix visual bug from excessive LIs being rendered around Category breadcrumbs

The category breadcrumb template in `templates/components/bread-crumbs.js.handlebars` contains LI tag wrappers around the references to the category-drop component (`components/categorydrop.component.js`). However, the component is already a LI itself, so the browser receives a HTML sequence `<li> <li>foo</li> </li>` from the Ember buffer. The first LI tag is automatically closed by the browser, leading to the rendered structure being `<li></li> <li>foo</li> ...`. This extra LI tag causes a visual issue, which by coincidence isn't visible on the default theme. See screenshot below.

![categorybreadcrumbli](https://f.cloud.github.com/assets/5316880/2480507/2b7a5122-b0bf-11e3-9b19-cd85302063dc.png)

Commit message:
Prevent rendering of excessive `<li>` wrapper around a category breadcrumb which already is a `<li>`.

The excessive `<li>` wrapper coming from the template is automatically closed by the browser during the HTML parsing step, causing an empty `<li>` being visible in the rendered source (preceding the first breadcrumb). This causes a visual bug when not using the default Discourse stylesheet, because the extra LI element pushes the content to the right and sometimes also renders an extra black pixel.