Rewrite integration tests for user-dropdown component to use page objects

Falls under umbrella issue #697.

## Resources

- What's a page object? [[1](http://martinfowler.com/bliki/PageObject.html),[2](https://wyeworks.com/blog/2015/5/13/using-the-page-object-pattern-with-ember-cli/)]
- [Quickstart guide](http://ember-cli-page-object.js.org/docs/v1.6.x/quickstart)
- See [`tests/pages/components/donations/create-donation.js`](https://github.com/code-corps/code-corps-ember/blob/develop/tests/pages/components/donations/create-donation.js) and [`tests/integration/components/donations/create-donation-test.js`](https://github.com/code-corps/code-corps-ember/blob/develop/tests/pages/components/donations/create-donation.js) for in-house examples of, respectively, a page object and the page object being utilized in an integration test.
- To generate a page object, run `ember generate page-object component/` in the project directory.