Refactor Submittable and Upload triggers

* Move `onUploadStart` & `onUploadStop` methods into the `Ajaxable` mixin so that they work with anything that triggers adding to the blocks ajax queue.
* Refactor `Submittable`, change to an object rather than a function that invokes an object.
* Scope the input to only find the submit btn inside of the form
* Remove the `setSubmitBtn` method from the `Image` block – it was unnecessary
* Add tests for Ajaxable & Submittable