Summary:
This adds a nuanced redirecting endpoint. For example, on
docs.getgrist.com it does:
1) If logged in and no team site -> https://docs.getgrist.com/
2) If logged in and has team sites -> https://docs.getgrist.com/welcome/teams
3) If logged out but has a cookie -> /login, then 1 or 2
4) If entirely unknown -> /signup
Test Plan: added a test; tested behavior through logins manually
Reviewers: dsagal
Reviewed By: dsagal
Subscribers: dsagal
Differential Revision: https://phab.getgrist.com/D3828
12f9567ff4