We want to wrap the `Ember.run.debounce` function and internally call `Ember.run` instead when running tests.
This commit changes discourseDebounce to work the same way as `Ember.run.debounce`.
Now that `discourseDebounce` works exactly like `Ember.run.debounce`, let's replace it and only use `DiscourseDebounce` from now on.
Move debounce to discourse-common to be able to reuse it in different bundles
Keep old debounce file for backwards-compatibility
