(Feature) Supplement "Add Mining Key" by extra fields to allow adding three keys at once

[varasev]

Problem: now VotingToChangeKeys smart contract allows to create ballot to add new validator by adding three keys at once (mining/voting/payout), but the web interface of Voting DApp doesn't allow to use this feature yet.

Solution: add an ability to create three validator's keys at once to the web interface of Voting DApp.

This issue is related to #83 and poanetwork/poa-network-consensus-contracts#92.

[varasev]

I've added extra fields for creating a new validator in varasev@661beb1.

This hasn't been tested yet with the new contracts, but web interface works fine. Now it looks like this:

[varasev]

This is how a ballot card looks:

[varasev]

Commits with the changes described above:

varasev@661beb1
varasev@4295932
varasev@944db87
varasev@8da4912
varasev@d075d8a

The corresponding PR will be created after security audit of smart contracts is done.

[varasev]

Done in #134.