wordpress: add SSO

Wordpress-plugin-http-authentication is packaged in Debian. Once it gets to stable, we should implement it in FreedomBox.

I tested the extension and it works well with mod_auth_tkt. The package also installs

/etc/apache2/conf-available/wordpress-plugin-http-authentication.conf containing the following lines:

<IfModule mod_rewrite.c>
        RewriteEngine On

        RewriteCond %{HTTP:Authorization} ^(.+)$
        RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
</IfModule>

The plugin however needs to be activated after installation which means we have to tap into the wordpress database.