(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/server/lib/requestUtils.ts

@@ -1,5 +1,5 @@
 import {ApiError} from 'app/common/ApiError';
-import {DEFAULT_HOME_SUBDOMAIN, isOrgInPathOnly, parseSubdomain} from 'app/common/gristUrls';
+import {DEFAULT_HOME_SUBDOMAIN, isOrgInPathOnly, parseSubdomain, sanitizePathTail} from 'app/common/gristUrls';
 import * as gutil from 'app/common/gutil';
 import {DocScope, QueryResult, Scope} from 'app/gen-server/lib/HomeDBManager';
 import {getUserId, RequestWithLogin} from 'app/server/lib/Authorizer';
@@ -70,7 +70,8 @@ export function addOrgToPath(req: RequestWithOrg, path: string): string {
  * Get url to the org associated with the request.
  */
 export function getOrgUrl(req: Request, path: string = '/') {
-  return getOriginUrl(req) + addOrgToPathIfNeeded(req, path);
+  // Be careful to include a leading slash in path, to ensure we don't modify the origin or org.
+  return getOriginUrl(req) + addOrgToPathIfNeeded(req, sanitizePathTail(path));
 }
 
 /**