`window` and `document` are easily injected anyhow, but this
primarily this keeps everything dom related in the same place.
Included here are changes to make all ember related objects use the dom
service `document` and `viewport` instead of just `document` and
`window`.
Quote from a previous PR (#4924) which explains the thinking around this:
> Now I have all these things in the dom service, it would make sense
to get window from there also. I was thinking of making a viewport
method, which would be a nice word whether window was a browser window,
an iframe (not really a window) like when ember testing, or anything
else. To me the viewport is what we are actually talking about here.
1. Split the resizing functionality of into a separate mixin to be
shared across components
2. Add basic integration tests to prove that everything is getting
called through out the lifetime of the app. I decided against unit
testing as there isn't really any isolated logic to be tested, more
checking that things are being called in the correct order etc i.e. the
integration is correct.
Adds assertion to with-resizing so its obvious to override `resize`
