Update for modern Ember

[chriskrycho]

The current implementation has worked well for a long time, but:

• it won't work with Ember 4, since the Ember global has been deprecated
• I think everything this needs to do in terms of lifecycle behavior (and probably quite a bit of the rest of it) can actually be done with public API now, via the router service events
• There's a fair bit of code in here that exists only to support Ember <2.0.0, which can also be safely dropped

An alternative approach might be: deprecate this in favor of the framework-supplied ember-page-title addon, which supports much of the same behavior.