Require python-bcrypt as a build-from-source dependency.
Finally fix firstboot redirect problems.

This issue was easy to solve once I started using the Live HTTP
Headers Firefox extension to debug it:

https://addons.mozilla.org/en-US/firefox/addon/live-http-headers/

With this particular InternalRedirect, CherryPy inexplicably responds
with a 301 ("Moved Permanently") unless we manually tell it to use a
307 ("Moved Temporarily") response code.  I don't know why CherryPy
does this, and it may be indicative of a deeper problem that I don't
currently have time to debug, as the request-response process for
redirecting from http://(server)/plinth to
https://(server)/plinth/firstboot is ridiculous and contains about
twice as many requests as I would've expected.