ux p2pool: enable 'start mining' if user inputs remote host flags

[plowsof]

we don't need to be connected to a local/remote node to start mining. if user enters these p2pool flags: --host node.monerodevs.org --rpc-port 18089 --zmq-port 18084, "start mining" should be possible.

clicking on "start mining" should grab the values currently inside the startup flags box (currently you have to stop/start for the new values to be used)

There is also no 'error: p2pool failed to start' screen (if user supplies bad arguments or the host ^ is offline / something else goes wrong.