[scikit-image] New chat forum

Hi everyone, I'd like to try Zulip for a while as a new chat forum for skimage. Gitter's interface is clunky, and it makes it hard to track various discussions, especially once they've intermingled. The new online chat is here: https://skimage.zulipchat.com (You'll see that the gitter history has been imported.) It takes a little bit of getting used to, but in essence its most useful feature is that you can start conversations on various "topics". Please sign in, especially during the upcoming Paris sprint, and let's see how it goes! Best regards, Stéfan