In `gh-content-view-container` the visibility of another DOM node was
being used to detect if a given view was mobile or not. This means the
UI needed to have layout forced (and DOM rendered) before the content
view container would render a second time. This is slow interaction with
the DOM (forcing layout) and slow for Ember's renderer (it needs to
render the container once with a default, then again when the value
changes).
Additionally there were two ways resize was being observed. The
`Window.matchMedia` API was used for some styles and the `ember-resize`
addon used to detect other changes. Here I've unified around just the
`Window.matcheMedia` API but abstracted it behind a service.
Sizes are exposed as properties that can be bound to or used directly in
templates.
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