Ability to run demo.magmo.com against other testnets

[andrewgordstewart]

Right now, demo.magmo.com only runs against Ropsten.

Ropsten had been down for 4 days prior to this morning, so it would be good to be able to switch testnets (or even use the mainnet!).

• Ideally, the user could select from a list of available networks.
• Minimally, we should know how to redeploy the app against another test network. In this case, we should instruct the user which network they should use, if they're on the incorrect network.