Add Localnet chain support

[matevz]

During the hackathons, we get a common request to have some kind of an explorer available to debug the transactions etc. Because of Oasis specifics and with the occurrence of ROFL, other explorers became infeasible.

We should enable people to run a complete Oasis Explorer locally (inside a docker image?) that connects to the sapphire-localnet or emerald-localnet images and indexes the chain. This should also include the Explorer's frontend.

Depends on oasisprotocol/oasis-web3-gateway#423