Update Web API templates to use new Web API conventions

[danroth27]

We are adding some new [Web API specific conventions]() that we will need the Web API project and time templates to react to.

We are adding [ApiController] attribute as the way to opt-in to new API specific conventions and behaviors that are tuned around best practices for APIs.

This will include:

• Automatically responding with a 400 when validation errors occur (using https://tools.ietf.org/html/rfc7807)
• Infer smarter defaults for action parameters ([FromBody] for complex types, [FromRoute] when possible, otherwise [FromQuery])
• Web API controllers should derive from ControllerBase (see https://github.com/aspnet/templating/issues/82)

[Eilon]

Added this to the list above:

• Web API controllers should derive from ControllerBase (see https://github.com/aspnet/templating/issues/82)

[Eilon]

@danroth27 / @rynowak / @pranavkm - should we get this into Preview1? Seems like a simple change that will help us get feedback on the greatness of [ApiController] and friends

[pranavkm]

With the exception of the first bullet not being fully fleshed out (we have the validation filter, we don't do RFC7807), there's enough in here to start getting some feedback.

[rynowak]

Should we add [ApiController]?

[danroth27]

I think we should