Package pre-built explorer and sandbox into the .deb artifact

[knazarov]

At the moment we ship source code of both explorer and sandbox to testnet machines and build them with npm there. It used to work well enough, but it's not the right approach to the problem.

Ideally we want both of them to be packaged and pre-built. In particular because we want to include them to nild binary (so that the user can get the whole complete environment when running nild without arguments)