IIRC the editor is being released with the game. Last we heard Steamworks would be used for distributing player made mods, though if there is any kind of content restrictions due to that, an alternate download site could be used (it just wouldn't get as much exposure as mods from the official source). Practically, I don't think pre-checking for offensive content would be reasonable (nobody is going to pay to have their free mod 'certified'), so there will likely just be some kind of user rating and feedback system. If there is adult content or potentially offensive material included, I'm sure most people would mention that upfront, anyway.
