Add a sample Selenium test for the login workflow

**TL;DR**: Add a sample Selenium test we can build on

**Narrative**
As a Developer, automated tests that cover Javascript and user interaction with a view will help me avoid introducing regressions and write more robust code.

**Acceptance Criteria**

- [ ] Selenium test using Chrome and/or Firefox to log in and register a child passes

No files or existing tests are named. Start by locating the project's browser-test setup and the login and child-registration entry points; the work is done when a Selenium test using Chrome and/or Firefox covers those flows and passes.

Written by the indexing model from the issue text.