Abstractions - Core MVC extensibility
Controllers - MVC implementations of .Abstractions and supporting
contracts
Infrastructure - General purpose support APIs. Metadata APIs that don't
fit clearly with a feature or with .Abstraction
- #2825
- new class names align with existing types such as `HttpNotFoundResult` and `HttpNotFoundObjectResult`
- remove similar types from WebApiCompatShim and use replacements in `ApiController`
- `NegotiatedContentResult<T>` remains because Core doesn't have an exact replacement
nits:
- add missing periods in some `Controller` doc comments