Revisions: Cleanup, bug fixes, refactoring, polish.

* Hide the tooltip initially.
* Fix a bug with routing.
* Further separate the Slider model and view, refactoring its code.
* More reliance on events than direct calls between areas.
* Smarter background diff loading (single mode). Loads the diffs closest to your position first.
* Removed a bunch of manual templating and `render()` methods. Now relies more on the WP Backbone Views functionality.
* include the requested `id` in `ensure:load`.
* new trigger: `ensure`, for `ensure()` attempts, regardless of whether they are already loaded.
* pass along a promise in both `ensure` and `ensure:load`.
* in `ensure`, remove requests for diffs we aready have

See #24425.

git-svn-id: https://core.svn.wordpress.org/trunk@24671 1a063a9b-81f0-0310-95a4-ce76da25c4cd