Get validation sample working
- add files needed to complete the sample
- implement rc2 (and later) `IClientModelValidator` interface
- remove validation TODOs; implement them for real
- remove unused `Movie` properties
- remove and sort `using`s
- adjust `validation.rst` to match new sample
- correct a couple of small typos
- use sample files instead of code blocks where possible
- addresses the remaining bits of #1154
350b3fc390