Corrected "Old Browser" warning: point to FireFox.

It's inappropriate for the FreedomBox project to recommend Non-Free
Software.  Therefore, we point users to the current version of FireFox
if their browser is too old.
Configure Apache correctly when installing Plinth.

Apache configuration was missing a few modules, and the server needs
to be restarted after those modules are enabled.  Also, the server
needs to be reloaded after the Plinth site is enabled.
Fix DocumentRoot: Apache can serve Plinth's static files.

Without this change, Apache tries to serve the static files from
/dev/null/static, which never exists, so users will see all exciting
manner of errors.

I've assumed that the correct directory is the default directory that
Freedom-Maker installs Plinth to, currently ``/home/fbx/plinth``.
Other distributions will need to patch this out to their preferred
location, like ``/usr/share/plinth/static``.