You must have the necessary prefix to create a build with an app ID. It
doesn't do anything yet, but in the future it will be used to control
access to other APIs.

For builds that have an app ID set, check that the presented build token
has that app prefix. This covers the build management endpoints, but not
yet build repo downloads or the status page.

Allow build repos to be set as public or private. If no app ID is set,
the default is public as before, but if an app ID *is* set, the default
is private (but in either case can be overridden). flat-manager-client gains
`--public_download`/`--no_public_download` flags to control this
behavior.

A private build requires a build token in order to download from it. A new scope
is added, "download", which allows listing and downloading builds.