Respect the `@cancellable` flag for `DirectServe{Html,Json}Resource`s (#12698)

`DirectServeHtmlResource` and `DirectServeJsonResource` both inherit
from `_AsyncResource`. These classes expect to be subclassed with
`_async_render_*` methods.

This commit has no effect on `JsonResource`, despite inheriting from
`_AsyncResource`. `JsonResource` has its own `_async_render` override
which will need to be updated separately.

Signed-off-by: Sean Quah <seanq@element.io>