(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/UserAPI.ts

@@ -333,7 +333,7 @@ export interface DocStateComparisonDetails {
 
 export interface UserAPI {
   getSessionActive(): Promise<ActiveSessionInfo>;
-  setSessionActive(email: string): Promise<void>;
+  setSessionActive(email: string, org?: string): Promise<void>;
   getSessionAll(): Promise<{users: FullUser[], orgs: Organization[]}>;
   getOrgs(merged?: boolean): Promise<Organization[]>;
   getWorkspace(workspaceId: number): Promise<Workspace>;
@@ -487,8 +487,8 @@ export class UserAPIImpl extends BaseAPI implements UserAPI {
     return this.requestJson(`${this._url}/api/session/access/active`, {method: 'GET'});
   }
 
-  public async setSessionActive(email: string): Promise<void> {
-    const body = JSON.stringify({ email });
+  public async setSessionActive(email: string, org?: string): Promise<void> {
+    const body = JSON.stringify({ email, org });
     return this.requestJson(`${this._url}/api/session/access/active`, {method: 'POST', body});
   }