Add POA Networks: Core and Sokol (#9413)

* ethcore: add poa network and sokol chainspecs

* rpc: simplify chain spec docs

* cli: rearrange networks by main/test and size/range

* parity: don't blacklist 0x00a328 on sokol testnet

* parity: add sokol and poanet to params and clean up a bit, add tests

* ethcore: add the poa networks and clean up a bit

* ethcore: fix path to poacore chain spec

* parity: rename poa networks to poacore and poasokol

* parity: fix configuration tests

* parity: fix parameter tests

* ethcore: rename POA Core and POA Sokol