e983f7bf3b
1. Add validation to BaseSvgScrapingService and update readthedocs accordingly. 2. Rewrite vso and add more tests. Rename it internally to azure-devops. URLs are still `/vso` for now. Should we make a way to let a service register multiple URL patterns? 3. Handle shared code using a functional pattern instead of inheritance. This comes from a discussion https://github.com/badges/shields/pull/2031#issuecomment-417893819. I like the functional approach because it's more direct, nimble, and easy to reason about; plus it allows services to grow from a family of one to two more easily.