Fix sync-related errors that show up in the console when not using a server (#984)

Previously, the frontend would attempt to make real requests to
`https://not-configured/`, which of course failed. I’ve changed the
internal structure to have the lack of a server expressed as a `null`
server. A new `did-bootstrap` browser-level setting has been added to
track if the user clicked “Don’t use a server.”

Finally, I updated the auth logic for methods that call out to the
server to make them consistent and use the early return style which
reduces indentation.