(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

test/nbrowser/Fork.ts

@@ -93,7 +93,7 @@ describe("Fork", function() {
           it(`can create an ${content} unsaved document when ${mode}`, async function() {
             let visitedSites: string[];
             if (mode === 'anonymous') {
-              visitedSites = ['Grist Templates'];
+              visitedSites = ['@Guest'];
               await personal.anon.login();
             } else {
               visitedSites = ['Test Grist', `@${personal.name}`];