(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/client/ui2018/cssVars.ts

@@ -788,6 +788,11 @@ export const theme = {
     undefined, colors.darkGrey),
   highlightedCodeBgDisabled: new CustomProp('theme-highlighted-code-bg-disabled',
     undefined, colors.mediumGreyOpaque),
+
+  /* Login Page */
+  loginPageBg: new CustomProp('theme-login-page-bg', undefined, 'white'),
+  loginPageBackdrop: new CustomProp('theme-login-page-backdrop', undefined, '#F5F8FA'),
+  loginPageLine: new CustomProp('theme-login-page-line', undefined, colors.lightGrey),
 };
 
 const cssColors = values(colors).map(v => v.decl()).join('\n');