bin: add build-dashboard for generating generated/dashboard/dashboard.go
bin: add build-dashboard for generating generated/dashboard/dashboard.go

In order for cored to serve the most recent dashboard code, dashboard assets (JavaScript and CSS bundles) must be built and packed into a .go file using `build-dashboard`. Assets are compiled into the Go binary as strings and served directly from memory.

Closes chain/chainprv#1476.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@dominic




chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain