Set up routing for app

[TheDizruptor]

Using BrowserRouter from react-router-dom, a switch needs to be put in place for switching between pages and authentication checks need to be added to prevent un-authenticated users from accessing todo lists. In the future it may be possible to have a local todo list without logging in with the ability to create an account and save it, but that's for a later version.

[TheDizruptor]

Routing and user authentication essentially complete, just needs some minor improvements to the styling and user experience.