[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.

This reverts the feature we added some months ago to eagerly update the URL even when a transition takes a while to complete; we realized that certain bugs (e.g. #5210 and #4908) didn't really otherwise have a clean solution since the original feature was at odds with the URL behavior people have come to expect from browsers.