UI: add buttons for dispute-level maintenance

[jaybuidl]

The Next Period button calls KC.passPeriod(dispute.id)
The Execute button calls KC.executeRuling(dispute.id)
The Distribute Rewards/Penalties button calls KC.execute(dispute.id, dispute.currentRoundIndex, 20)

If the prepareCall fails, make the corresponding button greyed out (disabled), possibly show the error message somewhere (in the console or in a tooltip?).