[FEATURE "ember-routing-transitioning-classes”]

Disables eager URL updates during slow transitions in favor of new CSS
classes added to `link-to`s (in addition to `active` class):

- `transitioning-in`: link-to is not currently active, but will be
  when the current underway (slow) transition completes.
- `transitioning-out`: link-to is currently active, but will no longer
  be active when the current underway (slow) transition completes.
Merge pull request #9919 from machty/transitioning-classes

[FEATURE "ember-routing-transitioning-classes”]