refs https://github.com/TryGhost/Team/issues/559
- switched to new ember-promise-modals pattern
- removed controller and template in favor of opening modals directly from the route
- removed unused `mousedown` event handlers - they are only necessary when an input blur would trigger validation errors
- fixed Enter key not triggering create action by adding an `{{on-key "Enter"}}` event handler to the name input
- fixed scroll not resetting to top of integrations screens when navigating between them by adding `{{scroll-top}}` element modifier to the main content sections
