Makes `pkg/web` only accept handles from the following set:
```go
handlerStd = func(http.ResponseWriter, *http.Request)
handlerStdCtx = func(http.ResponseWriter, *http.Request, *web.Context)
handlerStdReqCtx = func(http.ResponseWriter, *http.Request, *models.ReqContext)
handlerReqCtx = func(*models.ReqContext)
handlerReqCtxRes = func(*models.ReqContext) Response
handlerCtx = func(*web.Context)
```
This is a first step to reducing above set to only `http.Handler`.
---
Due to a cyclic import situation between `pkg/models` and `pkg/web`, parts of this PR were put into `pkg/api/response`, even though they definitely do not belong there. This however is _temporary_ until we untangle `models.ReqContext`.
