(core) Improve the look and behavior of /welcome/teams page (also shown for /welcome/start)

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

app/common/ThemePrefs-ti.ts

@@ -386,6 +386,9 @@ export const ThemeColors = t.iface([], {
   "highlighted-code-fg": "string",
   "highlighted-code-border": "string",
   "highlighted-code-bg-disabled": "string",
+  "login-page-bg": "string",
+  "login-page-backdrop": "string",
+  "login-page-line": "string",
 });
 
 const exportedTypeSuite: t.ITypeSuite = {