Make initial route redirect configurable

[conorsch]

Filing as follow-up to #80.

It's a nice touch to redirect the trading view to penumbra:gm on the testnet; however, that route should probably be configurable, too, maybe with the previous setting of penumbra:usdc retained as default behavior. Otherwise, deployments against the penumbra-1 chain will show nonsense data, because there's no gm asset.

Originally posted by @conorsch in #80 (review)