Related to #7206, this delegates the handling of the download to the browser
This changes how the enroll secret is sent to the server, as they might contain /, which was causing problems with our router.
Related to #6894, this entirely replaces FLEET_DEMO with the server config added in #6597 As part of this, I also implemented a small refactor to the integration test suite to allow setting a custom config when the server is initialized.
Rel: #6365, this adds a new endpoint to check and download pre-built installers.
