Summary:
- Move css module for the login page css to core/, to be reusable in core/ pages.
- Move /welcome/teams implementation to WelcomeSitePicker.ts
- List users for personal sites, as well as team sites.
- Add org param to setSessionActive() API method and end endpoint, to allow
switching the specified org to another user.
- Add a little safety to getOrgUrl() function.
Test Plan: Added a test case for the new behaviors of the /welcome/teams page.
Reviewers: georgegevoian
Reviewed By: georgegevoian
Differential Revision: https://phab.getgrist.com/D3914
