(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/server/lib/ITestingHooks-ti.ts

@@ -18,6 +18,7 @@ export const ITestingHooks = t.iface([], {
   "flushAuthorizerCache": t.func("void"),
   "getDocClientCounts": t.func(t.array(t.tuple("string", "number"))),
   "setActiveDocTimeout": t.func("number", t.param("seconds", "number")),
+  "setDiscourseConnectVar": t.func(t.union("string", "null"), t.param("varName", "string"), t.param("value", t.union("string", "null"))),
 });
 
 const exportedTypeSuite: t.ITypeSuite = {