Simplify SnapshotsController

In 1972a90c2bf09ecb3b8d7 we removed the JSON-response support that Rails
auto-generated as part of initializing the Url model. We didn't need it
there, and we don't need it here, in SnapshotsController.

Change-Id: I2af4d2c97cbeb1b1f0559dc1915c4ff96ee0d1b7