Fix Manage Validator Ballots page after HF in Core (in November 2018)

[varasev]

We should rewrite Adding a Validator key / Use Case 1 and Ballot Finalization Order sections on Manage Validator Ballots page after HF in Core which is scheduled for November 17, 2018.

The new smart contracts and Governance DApp allow to add a new Validator with three keys at once in the same ballot (mining key, voting key, payout key), so we should update descriptions mentioned above accordingly.