(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

test/nbrowser/testServer.ts

@@ -100,6 +100,8 @@ export class TestServerMerged implements IMochaServer {
       // Set low limits for uploads, for testing.
       GRIST_MAX_UPLOAD_IMPORT_MB: '1',
       GRIST_MAX_UPLOAD_ATTACHMENT_MB: '2',
+      // The following line only matters for testing with non-localhost URLs, which some tests do.
+      GRIST_SERVE_SAME_ORIGIN: 'true',
       APP_UNTRUSTED_URL : "http://localhost:18096",
       // Run with HOME_PORT, STATIC_PORT, DOC_PORT, DOC_WORKER_COUNT in the environment to override.
       ...(isCore ? {