Client-side auth0 integration

[bion]

Client-side work for https://github.com/distributeaid/toolbox/issues/18

Removes AWS Amplify code and replaces it with Auth0 in the browser client.

Manually confirmed that:

• can log in
• can log out
• user remains logged in after refreshing the page
• the client can get a JWT from Auth0 and include it as a Bearer: [TOKEN] header in requests to the toolbox server

Also fixed a couple warnings from react about not providing unique keys for enumerated components.