Handle case where selected IdP is not available

[baszoetekouw]

If the currently selected IdP is not connected to the service the user is accessing, the extension will still submit the user's preferred IdP to Engineblock. This leads to an error.

Fix this by checking whether the selected IdP is present in the connectedIdps array in the wayf-configuration json. If the preferred IdP is not present, show a warning (top bar overlay or something liek that) but don't automatically submit an IdP.