(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/themes/GristDark.ts

@@ -483,4 +483,9 @@ export const GristDark: ThemeColors = {
   'highlighted-code-fg': '#A4A4A4',
   'highlighted-code-border': '#69697D',
   'highlighted-code-bg-disabled': '#555563',
+
+  /* Login Page */
+  'login-page-bg': '#32323F',
+  'login-page-backdrop': '#404150',
+  'login-page-line': '#57575F',
 };

app/common/themes/GristLight.ts

@@ -483,4 +483,9 @@ export const GristLight: ThemeColors = {
   'highlighted-code-fg': '#929299',
   'highlighted-code-border': '#D9D9D9',
   'highlighted-code-bg-disabled': '#E8E8E8',
+
+  /* Login Page */
+  'login-page-bg': 'white',
+  'login-page-backdrop': '#F5F8FA',
+  'login-page-line': '#F7F7F7',
 };