Allow user to edit status and notes in place.

* Create an array of statuses.
* The 'click' attribute in the form specifies a function to call when user click the submit
button. Ember expects the controller to define this function, that must be within 'actions'
since it's being called from a template action.
* The controller should be an ObjectController because it wraps a single lead model.
* save() will actually send an AJAX PUT request to our Rails API, with 'api/v1/leads/<id>'.
Prompt the user to save changes before switching to a different lead.

* More info: https://github.com/vicramon/ember-tutorial/blob/master/chapters/context-did-change.md
Remove 'model' in templates, ObjectController will proxy the model.
Allow user to edit leads info.

* Since the 'route' is nested inside a 'resource', Ember expects the
template to be inside a subdirectory with the name of the resource.
* The 'edit' route is nested under 'lead' so Ember renders the template
into an 'outlet' tag inside the 'lead' template.
* Set an 'isEditing' property on the controller to false by default.