Enable 'Digital collections' link on home page.

There isn't an access point specified in the mocks for the search of all collections AFAICT so this PR simply enables the link on the home page and fills out the appropriate parameters.

![digital-collections](https://cloud.githubusercontent.com/assets/96776/3127425/2d3d8aac-e7be-11e3-8739-ea09bf01a04f.png)