Document changes.
Cleanup Plinth's Apache configuration

- Plinth related SSL redirection is no longer required in fbx.conf.

- Plinth no-longer hijacks default SSL site configuration.  Enable
  default SSL site configuration to compensate.

- Plinth's deault configuration works just fine, don't disable it.

- The change is need for the latest Plinth's Apache configuration
  changes at https://github.com/freedombox/Plinth/pull/25

- Newer version of FreedomBox setup should depend on Plinth > 0.4.1.
Set HSTS header on default SSL site

- As Plinth is no longer responsible for managing SSL site
  configuration.

- The HSTS header should not be set for non-secure requests as per RFC.

- Enable for all sub-domains also for slightly better security.  All
  subdominas for FreedomBox are likely to have SSL as well and are
  likely to be configured using freedombox-setup.