Remove saved search digest form if account has no searches [PD-523]

Replaces the saved search digest form with a short blurb about digests and a note that it will be enabled once the account has at least one saved search.

All tests pass.

No searches:
![2014-10-03-140812_688x334_scrot](https://cloud.githubusercontent.com/assets/3379265/4510242/6d4a8762-4b28-11e4-8200-af93ab235982.png)

One or more search:
![2014-10-03-141034_976x532_scrot](https://cloud.githubusercontent.com/assets/3379265/4510260/95a42fe2-4b28-11e4-84a9-752106357df9.png)