Add network hashrate chart

[jzbz]

Currently https://explorer.dcrdata.org/charts has PoW difficulty but not network hashrate. Having the latter would be useful as well.

[chappjc]

I started looking into this today. We have a CalculateHashRate function that does the computation based on difficulty and target block time. However, the implementation in dcrd's getnetworkhashps is very different, accumulating work over a variable period of blocks. The hander is defined here: https://github.com/decred/dcrd/blob/7e3dfb21974681090ad703c2350fe8146f14aa08/rpcserver.go#L3154

[xaur]

Hashrate will be derived solely from chain data, not using anything that pools report -- correct?

[chappjc]

Absolutely. Only based on work done on the blockchain over a given period

[oregonisaac]

Ping: suggesting we move forward with adding current hash rate based on the block difficulty and the default past 120 (or some other stated number) blocks using RPCs in dcrd.

[chappjc]

@buck54321 has a proper solution underway

[chappjc]

Resolved by #808