Enable SCSS for guest pages

SCSS is disabled for several types of pages, as the SCSS compiler uses
the cache system, which is not available for example when Nextcloud is
not installed yet. However, in regular guest pages the cache system is
available, so SCSS is now enabled for them too.

To keep the same behaviour for guest pages the guest style is
automatically added for them, as before this commit it was automatically
added due to SCSS not being used.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>