Override server SCSS rules for confirm button in guest pages

As the server defines some SCSS rules using "XXX + .icon-confirm" the
selector is more specific than just "input" used in "guest.css", so
the server SCSS rules took precedence over the guest CSS rules. Now a
more specific selector was added to the guest and public share auth
rules; as the guest and public share auth rules are loaded after the
server ones now the guest rules for input elements take precedence over
the server ones.

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