add config option to show SPID button disabled

[simevo]

all CMS plugins / framework bundles that want to be dynamically configurable can find themselves in a state where they have been installed and activated, but the configuration is incomplete (typically because the SP certificates are missing); see for example https://github.com/simevo/spid-wordpress/issues/22

in this state the plugin / bundle when requested to show the SPID button should do that, but the SPID button should be in a disabled state: grayed out and doesn't react on click