[FEATURE ember-router-loading-error-substates] flaggify substates

Much of the internal implementation for the Router had to change
to allow for substates, hence my reluctance to throw feature flags
on the previous substates commit, but we do in fact want/need
feature flags, so here they are. When the feature flag is disabled,
legacy (inconsistent/kind of broken) LoadingRoute behavior is
preserved.
Merge pull request #3617 from machty/loading-substates

[FEATURE ember-router-loading-error-substates] flaggify substates