To be able to write to a person's Pod, they need to log in and
authorise our app. To do this, we use the React components in
https://www.npmjs.com/package/@solid/react.

Although it unfortunately comes with a bunch of stuff we will not
be using, the authentication components are easy enough to use:
wrap components that should be visible to people who are not
logged in inside a <LoggedOut> component, and those that should
only be visible to those who are logged in inside a <LoggedIn>.

Actual authentication is initiated with a <LoginButton>.
Unfortunately, at this time this involves including an HTML blob
verbatim as downloaded from
https://solid.github.io/solid-auth-client/dist/popup.html
(this is linked to from the README of @solid/react). In the future,
this will hopefully be properly packaged.