(core) Revert part of the recent tweaks to AppHeader

Summary:
In non-SaaS Grist, the AppHeader will no longer link to templates
page. Also, the last visited site is now reset when switching accounts.

Test Plan: Existing tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D4048

app/client/models/AppModel.ts

@@ -128,6 +128,7 @@ export interface AppModel {
   isOwnerOrEditor(): boolean;           // If user is an owner or editor of this org
   /** Creates an computed observable to dismiss a popup or check if it was dismissed */
   dismissedPopup(name: DismissedPopup): Observable<boolean>;
+  switchUser(user: FullUser, org?: string): Promise<void>;
 }
 
 export class TopAppModelImpl extends Disposable implements TopAppModel {
@@ -402,6 +403,11 @@ export class AppModelImpl extends Disposable implements AppModel {
     return computed;
   }
 
+  public async switchUser(user: FullUser, org?: string) {
+    await this.api.setSessionActive(user.email, org);
+    this.lastVisitedOrgDomain.set(null);
+  }
+
   private _updateLastVisitedOrgDomain({doc, org}: IGristUrlState, availableOrgs: Organization[]) {
     if (
       !org ||