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
