Fix app.delete to return post to res.send(data)

[dglmartins]

I think the readable server code has a bug on app.delete where the post returned by comments.disableByParent is not returned to the next app.delete then, and as such, there is a error when calling res.json() when fetching. This can cause confusion with students that are trying to use the returned post or that have written the fetch call with res => res.json(). Inside app.delete in the server, changing
.then( (post) => { comments.disableByParent(req.token, post) })

(which returns nothing)
to
.then( (post) => comments.disableByParent(req.token, post) )

or to
.then( (post) => { return comments.disableByParent(req.token, post) })

fixes the bug, returning the post as datato .then((data) => res.send(data),