Returning a promise from addEvent action
So as I said before that the idea here was that we won't actually mutate the state, in
other words we won't commit this addEvent in our action until we get a successful response
from the server.
So what we're going to do is I'm going to put a then, because Axios methods return a
promise and so therefore you can chain a then on to the end of it and the argument that
you get, you get the response and so we'll create a callback function inside the then