This adds a toggle to disable gradients, fixing #140.
On the Mandala page, this changes how our permalinks are structured, but does so in a backwards-compatible way, thanks to [Avro schema evolution][1]. Now the `s` querystring argument is prefixed with a schema version, the latest being `v2`. If this prefix is missing, it's assumed to be the very first schema version, `v1`, and interpreted accordingly.
[1]: https://martin.kleppmann.com/2012/12/05/schema-evolution-in-avro-protocol-buffers-thrift.html
Atul Varma