from the index page, render the search_sidebar partial 

instead of the facets partial directly to give applications a hook to modify the sidebar without overriding the whole index template.


We have a couple cases in searchworks where we'd like to prepend or append content to the sidebar depending on the search context, e.g:

http://searchworks.stanford.edu/?f%5Bformat%5D%5B%5D=Database

http://searchworks.stanford.edu/?f%5Bcourse%5D%5B%5D=AA-100-01&f%5Binstructor%5D%5B%5D=Pavone%2C+Marco

![screen shot 2013-09-23 at 11 43 44](https://f.cloud.github.com/assets/111218/1194122/1c27079a-2480-11e3-9484-94b454d106a0.png)
![screen shot 2013-09-23 at 11 43 51](https://f.cloud.github.com/assets/111218/1194123/1c2ea8a6-2480-11e3-9f65-a392156d4bba.png)

Or, in some other sides, render some other partials:

![screen shot 2013-09-23 at 11 44 19](https://f.cloud.github.com/assets/111218/1194131/350fc404-2480-11e3-9a43-1de92706eeb4.png)

This commit allows us to override just the sidebar partial, instead of the whole index template.