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.

* Allow the server to auto-configure the server URL for the client

* Extract server URL/version logic out to ensure consistent updates

* ()

* Be more explicit about when the server version is re-fetched

* Use a single layer of context provider

* Move the bootstrap route to /account/needs-bootstrap

* No more `isActual`

* Refactor to call subscribe-needs-bootstrap instead of fetch()

* Dedupe calls to subscribe-needs-bootstrap

* Don’t revalidate the server when we just validated it

* simplify

* Fix setServerURL