PORTLAND, Maine — Before Graham Platner dove into the meat of his speech at the Maine Democratic Party Convention Saturday night, where he said it is “very odd but deeply humbling” to be the “presumptive nominee” for Senate, he first thanked his former opponent, Gov. Janet Mills. A smattering of party delegates rose to their feet and applauded. Jens Dill was one of them. A 72-year-old retired software engineer from Winthrop, he had long been behind Platner in the primary ahead of a tough race