cmd/dcrdata: dynamically scale hashrate units

[ttm-cmd]

Closes #1987

This updates hashrate displays to dynamically select an appropriate unit instead of always showing Ph/s.

The hashrate chart and homepage hashrate card now display Th/s, Ph/s, or Eh/s depending on the hashrate value. Homepage websocket updates also refresh the unit.

Verification:

node_modules\.bin\eslint.cmd public --ext .js --ext .jsx --quiet
node_modules\.bin\webpack.cmd --config webpack.prod.js --progress --color
gofmt -w cmd/dcrdata/internal/explorer/templates.go