Add searchable options to appearances page.

Closes #785 

Add warning to the Browse admin page when an exhibit is not searchable.

Remove facets and searchbox on home page if an exhibit is not searchable.

### Appearances page
![checkbox](https://cloud.githubusercontent.com/assets/96776/5324506/1cc8d3f4-7c90-11e4-9ba6-00e89320463f.png)

### Browse page warning
![browse-warning](https://cloud.githubusercontent.com/assets/96776/5324505/1cc891e6-7c90-11e4-9334-537da3a2bdbb.png)

### Home page
![home-page](https://cloud.githubusercontent.com/assets/96776/5324504/14300b7c-7c90-11e4-9699-90ab1c9ee3b5.png)

Sneaking this in here now  so we can do #786 w/o an additional migration.