Polls closed on Thursday following UK local and regional elections that could prove historic in Northern Ireland and heap further pressure on embattled Prime Minister Boris Johnson. The contest for the devolved assembly in Belfast could see a pro-Irish nationalist party win for the first time in the troubled history of the British province. The results, expected from Friday, could have huge constitutional implications for the four-nation UK’s future, with predicted victors Sinn Fein committed to a vote on reunification with Ireland. Voters were electing councils in Scotland, Wales and much of …