When Elizabeth Warren announced that big-money donors won’t get special access to her campaign, it was a gamble, but not as risky a move as it would have been just a few years ago. Over the last few election cycles, small-dollar donations have become a staple, especially for Democrats. And as Adam Reilly tells us that shift in the way campaign finance works can be traced, in large part, to an outfit based in Somerville’s Davis Square.