Apache configuration fixes

The motivation for this change is issue #4 and the [bug](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=769328) that make Plinth unusable after install[1].

I have pretty much rewritten the Apache configuation file for Plinth with the following main highlights:

- Don't hijack the entire SSL site for sake of Plinth.

- Serve both HTTP and HTTPS sites with default configuration.

- plinth-ssl configuration only makes SSL compulsory.

- Don't disable default site during FreedomBox setup run.  This is no
  longer required.

- Setup and use X-Forwarded-Proto and X-Forwarded-Host headers.

- Allow Plinth to be used with any Host: header since hostname of FreedomBox machine could be anything and could change.

- Allow full set of private IP addresses to access Plinth.