(core) Implement DiscourseConnect to enable easy sign-in to community forum

Summary:
- Update cookie module, to support modern sameSite settings
- Add a new cookie, grist_sid_status with less-sensitive value, to let less-trusted subdomains know if user is signed in
- The new cookie is kept in-sync with the session cookie.
- For a user signed in once, allow auto-signin is appropriate.
- For a user signed in with multiple accounts, show a page to select which account to use.
- Move css stylings for rendering users to a separate module.

Test Plan: Added a test case with a simulated Discourse page to test redirects and account-selection page.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3047

app/client/aclui/ACLUsers.ts

@@ -2,7 +2,8 @@ import {copyToClipboard} from 'app/client/lib/copyToClipboard';
 import {DocPageModel} from 'app/client/models/DocPageModel';
 import {urlState} from 'app/client/models/gristUrlState';
 import {createUserImage} from 'app/client/ui/UserImage';
-import * as um from 'app/client/ui/UserManager';
+import {cssMemberImage, cssMemberListItem, cssMemberPrimary,
+        cssMemberSecondary, cssMemberText} from 'app/client/ui/UserItem';
 import {basicButton, basicButtonLink} from 'app/client/ui2018/buttons';
 import {colors, testId} from 'app/client/ui2018/cssVars';
 import {icon} from 'app/client/ui2018/icons';
@@ -24,14 +25,14 @@ const roleNames: {[role: string]: string} = {
 function buildUserRow(user: UserAccessData, currentUser: FullUser|null, ctl: IOpenController) {
   const isCurrentUser = Boolean(currentUser && user.id === currentUser.id);
   return cssUserItem(
-    um.cssMemberImage(
+    cssMemberImage(
       createUserImage(user, 'large')
     ),
-    um.cssMemberText(
-      um.cssMemberPrimary(user.name || dom('span', user.email),
+    cssMemberText(
+      cssMemberPrimary(user.name || dom('span', user.email),
         cssRole('(', roleNames[user.access!] || user.access, ')', testId('acl-user-access')),
       ),
-      user.name ? um.cssMemberSecondary(user.email) : null
+      user.name ? cssMemberSecondary(user.email) : null
     ),
     basicButton(cssUserButton.cls(''), icon('Copy'), 'Copy Email',
       testId('acl-user-copy'),
@@ -84,7 +85,7 @@ const cssUsers = styled('div', `
   max-width: unset;
 `);
 
-const cssUserItem = styled(um.cssMemberListItem, `
+const cssUserItem = styled(cssMemberListItem, `
   width: auto;
   padding: 8px 16px;
   align-items: center;