Symlink from app/views/catalog to app/views/blacklight/catalog...

This allows downstream applications (and Blacklight plugins/engines that override partials) to render blacklight-provided partials, even if they have been overridden elsewhere in an application.

This provides us with a couple features:
- applications that override blacklight partials can render the very partial they've overridden, e.g.:

```erb
Some customization
<%= render :partial => 'blacklight/catalog/did_you_mean' %>
Some more customization
```

- engines that override blacklight partials can explicitly render blacklight-provided partials (when, say, they really want Blacklight's version of catalog/document)