feat: Add Authenticator and SimpleAuthenticator

[travis]

add "headless" Authenticator component and use it to build a "customizable" SimpleAuthenticator that works the way the Authenticator components in our examples work

separate PR against feat/w3ui-ui to make feedback easier - this component will replace the Authenticator components we've copied into a number of different examples

this is the last major React "customizable" UI component - if we're happy with this we should probably push forward with Vue and SolidJS versions of each of them - hopefully more straightforward now that the architecture/design work has been done

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Sandbox	Source
@w3ui/example-react-file-upload	Configuration
@w3ui/example-react-sign-up-in	Configuration
@w3ui/example-react-uploads-list	Configuration
@w3ui/example-solid-file-upload	Configuration
@w3ui/example-solid-sign-up-in	Configuration
@w3ui/example-solid-uploads-list	Configuration
@w3ui/example-vue-file-upload	Configuration
@w3ui/example-vue-sign-up-in	Configuration
@w3ui/example-vue-uploads-list	Configuration

[travis]

I'm gonna merge this one too since it will be reviewed as part of #136 🤠