Ensure that `content` -> `model` alias does not break SemVer.

The ControllerContentModelAliasDeprecation mixin is used to provide a useful
deprecation warning when specifying `content` directly on a `Ember.Controller`
(without also specifying `model`).

Ember versions prior to 1.7 used `model` as an alias of `content`, but due to
much confusion this alias was reversed (so `content` is now an alias of `model).

This change reduces many caveats with model/content, and also sets a
simple ground rule: Never set a controllers content, rather always set
it's model and ember will do the right thing.