refs https://github.com/TryGhost/Ghost/issues/14101
- migrated staff user controller to native class syntax
- removed use of `{{action}}` helper
- moved from custom components to native `<input>` and `<textarea>` for form fields
- added `{{select-on-click}}` modifier to cover the `<GhTextingInput @selectOnClick>` option behaviour for any input element
- added `submitForm()` test helper that finds closest `form` element and trigger's a `submit` event on it simulating <kbd>Enter</kbd> being pressed whilst a field has focus
