(core) Adding GristConnect login system

Summary:
New login system to allow simple SSO flow that is based on Discourse description that is available at:
https://meta.discourse.org/t/discourseconnect-official-single-sign-on-for-discourse-sso/13045

Test Plan: New core test.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3418

app/client/ui/AccountWidget.ts

@@ -91,7 +91,7 @@ export class AccountWidget extends Disposable {
     }
 
     const users = this._appModel.topAppModel.users;
-
+    const isExternal = user?.loginMethod === 'External';
     return [
       cssUserInfo(
         createUserImage(user, 'large'),
@@ -138,7 +138,7 @@ export class AccountWidget extends Disposable {
             cssOtherEmail(_user.email, testId('usermenu-other-email')),
           );
         }),
-        menuItemLink({href: getLoginUrl()}, "Add Account", testId('dm-add-account')),
+        isExternal ? null : menuItemLink({href: getLoginUrl()}, "Add Account", testId('dm-add-account')),
       ],
 
       menuItemLink({href: getLogoutUrl()}, "Sign Out", testId('dm-log-out')),

app/client/ui/errorPages.ts

@@ -1,5 +1,5 @@
 import {AppModel} from 'app/client/models/AppModel';
-import {getLoginUrl, urlState} from 'app/client/models/gristUrlState';
+import {getLoginUrl, getMainOrgUrl, urlState} from 'app/client/models/gristUrlState';
 import {AppHeader} from 'app/client/ui/AppHeader';
 import {leftPanelBasic} from 'app/client/ui/LeftPanelCommon';
 import {pagePanels} from 'app/client/ui/PagePanels';
@@ -24,6 +24,8 @@ export function createErrPage(appModel: AppModel) {
  * Creates a page to show that the user has no access to this org.
  */
 export function createForbiddenPage(appModel: AppModel, message?: string) {
+  const isAnonym = () => !appModel.currentValidUser;
+  const isExternal = () => appModel.currentValidUser?.loginMethod === 'External';
   return pagePanelsError(appModel, 'Access denied', [
     dom.domComputed(appModel.currentValidUser, user => user ? [
       cssErrorText(message || "You do not have access to this organization's documents."),
@@ -32,12 +34,14 @@ export function createForbiddenPage(appModel: AppModel, message?: string) {
     ] : [
       // This page is not normally shown because a logged out user with no access will get
       // redirected to log in. But it may be seen if a user logs out and returns to a cached
-      // version of this page.
+      // version of this page or is an external user (connected through GristConnect).
       cssErrorText("Sign in to access this organization's documents."),
     ]),
     cssButtonWrap(bigPrimaryButtonLink(
-      appModel.currentValidUser ? 'Add account' : 'Sign in',
-      {href: getLoginUrl()},
+        isExternal() ? 'Go to main page' :
+        isAnonym() ? 'Sign in' :
+        'Add account',
+      {href: isExternal() ? getMainOrgUrl() : getLoginUrl()},
       testId('error-signin'),
     ))
   ]);