(core) Tweak navbar, breadcrumbs, and sign-in buttons

Summary:
The changes are intended to smooth over some sharp edges when a signed-out user
is using Grist (particularly while on the templates site).

Test Plan: Browser tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3957

app/client/ui/WelcomePage.ts

@@ -50,7 +50,7 @@ export class WelcomePage extends Disposable {
         panelWidth: Observable.create(this, 240),
         panelOpen: Observable.create(this, false),
         hideOpener: true,
-        header: dom.create(AppHeader, '', this._appModel),
+        header: dom.create(AppHeader, this._appModel),
         content: null,
       },
       headerMain: [cssFlexSpace(), dom.create(AccountWidget, this._appModel)],