sso: Need to enable mod auth_pubtkt

When testing plinth v0.15.1, I saw that installed web apps were not available at the expected locations. I tried reloading the apache conf, and saw the following error:

Jul 02 14:46:34 freedombox systemd[1]: Reloading The Apache HTTP Server.
Jul 02 14:46:34 freedombox apachectl[11604]: AH00526: Syntax error on line 1 of /etc/apache2/includes/freedombox-single-sign-on.conf:
Jul 02 14:46:34 freedombox apachectl[11604]: Invalid command 'TKTAuthPublicKey', perhaps misspelled or defined by a module not included in the server configuration

I think we need to enable this apache module during sso module setup.