Consensus blocks api

Mini real-time blocks explorer.

Since internal block explorer unusable, we need at least possibility to get 'raw' blocks from siad. I know that explorer have similar function, but:

1. Explorer unusable, very long and incorrect DB build. #1990 (closed)
2. Explorer block api unavailable with disabled explorer module(you dont say.jpg).

Also i believe that blocks api should be in consensus module.