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/settings/oidc-config/main.js

@@ -0,0 +1,47 @@
+const Mn       = require('backbone.marionette');
+const App      = require('../../main');
+const template = require('./main.ejs');
+
+require('jquery-serializejson');
+require('selectize');
+
+module.exports = Mn.View.extend({
+    template:  template,
+    className: 'modal-dialog',
+
+    ui: {
+        form:     'form',
+        buttons:  '.modal-footer button',
+        cancel:   'button.cancel',
+        save:     'button.save',
+    },
+
+    events: {
+        'click @ui.save': function (e) {
+            e.preventDefault();
+
+            if (!this.ui.form[0].checkValidity()) {
+                $('<input type="submit">').hide().appendTo(this.ui.form).click().remove();
+                return;
+            }
+
+            let view = this;
+            let data = this.ui.form.serializeJSON();
+            data.id  = this.model.get('id');
+            if (data.meta.enabled) {
+                data.meta.enabled = data.meta.enabled === "on" || data.meta.enabled === "true";
+            }
+
+            this.ui.buttons.prop('disabled', true).addClass('btn-disabled');
+            App.Api.Settings.update(data)
+                .then(result => {
+                    view.model.set(result);
+                    App.UI.closeModal();
+                })
+                .catch(err => {
+                    alert(err.message);
+                    this.ui.buttons.prop('disabled', false).removeClass('btn-disabled');
+                });
+        }
+    }
+});