Extend T377144: Create method for deterministically opting new users into SUL3 rollout so instead of a deterministic true/false site configuration flag, we can set a probability.
This is tricky because it kinda sucks if the user can get reassigned when they reload the signup page. If we punt on that problem, it's not the end of the world, but it would be nice to make the choice sticky for the signup session. Use IP hash as the random seed? Redirect immediately to an URL with a usesul3= flag? Set a short-term cookie?