Joystream-node embed mainnet chainspec

[mnaamani]

Embed joy-mainnet.json chainspec file in joystream-node binary to make it simpler to get started with running a node for mainnet. So now only need to download binary, and then run a mainnet node like this:

joystream-node --chain joy-mainnet

rather than needing to separately download the joy-mainnet.json file and running with:

joystream-node --chain /path/to/joy-mainnet.json