(core) Site Switcher and Other Sites

Summary:
A new section, Other Sites, will now be shown on the All Documents page when:

 - A user is on a personal site and has access to other team sites.
 - A user is on a public site with view access only.

In addition, a site switcher is now available by clicking
the site name in the top-left section of the UI next to the
Grist logo. It works much like the switcher in the Account
menu.

Test Plan: Browser tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2979

app/client/ui/WelcomePage.ts

@@ -4,7 +4,7 @@ import { submitForm } from "app/client/lib/uploads";
 import { AppModel, reportError } from "app/client/models/AppModel";
 import { getLoginUrl, getSignupUrl, urlState } from "app/client/models/gristUrlState";
 import { AccountWidget } from "app/client/ui/AccountWidget";
-import { appHeader } from 'app/client/ui/AppHeader';
+import { AppHeader } from 'app/client/ui/AppHeader';
 import * as BillingPageCss from "app/client/ui/BillingPageCss";
 import * as forms from "app/client/ui/forms";
 import { pagePanels } from "app/client/ui/PagePanels";
@@ -73,7 +73,7 @@ export class WelcomePage extends Disposable {
         panelWidth: Observable.create(this, 240),
         panelOpen: Observable.create(this, false),
         hideOpener: true,
-        header: appHeader('', this._appModel.topAppModel.productFlavor),
+        header: dom.create(AppHeader, '', this._appModel),
         content: null,
       },
       headerMain: [cssFlexSpace(), dom.create(AccountWidget, this._appModel)],