FEAT: Add Open ID Connect authentication method

* add `oidc-config` setting allowing an admin user to configure parameters
* modify login page to show another button when oidc is configured
* add dependency `openid-client` `v5.4.0`
* add backend route to process "OAuth2 Authorization Code" flow
  initialisation
* add backend route to process callback of above flow
* sign in the authenticated user with internal jwt token if internal
  user with email matching the one retrieved from oauth claims exists

Note: Only Open ID Connect Discovery is supported which most modern
Identity Providers offer.

Tested with Authentik 2023.2.2 and Keycloak 18.0.2

frontend/js/app/api.js

@@ -59,6 +59,8 @@ function fetch(verb, path, data, options) {
             },
 
             beforeSend: function (xhr) {
+                // allow unauthenticated access to OIDC configuration
+                if (path === "settings/oidc-config") return;
                 xhr.setRequestHeader('Authorization', 'Bearer ' + (token ? token.t : null));
             },