We do everything in issues and design is not an exception. When we have idea we create an issue and then someone from UX team comes up with screenshot/mockup of it. Mockups and feedback are posted as comments in the issue. It continues up until approval either from product person or other designer. In the end we have issue with description of what should be done and final screenshot attached.
There is also UX channel in Slack where UX team discuss new techniques and approaches.