GoReact is a video annotation site designed to be used for public speaking classes. It allows you to add time-stamped comments to videos that students upload. It also allows peer commenting and powerful class management tools (and may integrate into your LMS).
You can sign up for a free (until June 30) account by affiliating with the "University of Alaska - Anchorage - Seawolf Debate" license when asked to choose your organization.
GoReact also has a quick introduction to using their site, as well as complete documentation for instructors and students.
Dr. Steve Llano of St. John's University publishes the blog NYC Sophist. He has some very thoughtful posts about hosting debates online (that in many ways inspired what I recommend).
Rubrics: these are some selected evaluation rubrics that you may use or modify as you see fit.
From Teachers Pay Teachers (pay site)
From Study.com (pay site)
From NIU (free)
From WordGeneration (free)
Topics: If you'd prefer a larger selection of topics, you'll find some good ideas here.
The TopicsMill has a list of 70 good motions for middle school debates.
The New York Times has a list of 100 topics with links to backgrounders that provide a concise introduction to the topic.
Kialo is a site that allows students to work collaboratively to develop arguments for and against a controversial topic.