(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.ts

@@ -14,4 +14,5 @@ export interface ITestingHooks {
   flushAuthorizerCache(): Promise<void>;
   getDocClientCounts(): Promise<Array<[string, number]>>;
   setActiveDocTimeout(seconds: number): Promise<number>;
+  setDiscourseConnectVar(varName: string, value: string|null): Promise<string|null>;
 }