(Tries to) add reviewer roulette via Danger. The idea is that danger can scan the MR and determine what reviews it needs. Once decided, it can randomly pick a reviewer and maintainer for each category, perhaps taking OOO status into account along with any other information. It could even take CODEOWNERS information into account.
The final choice of which reviewers to use should remain with the author of the merge request - these are just suggestions to ease selection when the author doesn't have a strong opinion.
