Step 1 should be talking to affected parties, not building software. Meet and talk in person. When something happens, people can take notes on paper and when you meet you combine those notes of recorded incidents. This creates more of a community feeling than anonymous flyers with a link to a website.

Edit: The problem is armchair activism. Your proposed solution is moving the conversation from Facebook to your neighborhood platform. But in the end, if everyone keeps sitting in isolation in front of their computer, the problem remains. If you are not willing to talk to people now, you won’t be later, and not change a thing.