(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/common/gristUrls.ts

@@ -80,6 +80,7 @@ export const commonUrls = {
   plans: "https://www.getgrist.com/pricing",
   sproutsProgram: "https://www.getgrist.com/sprouts-program",
   contact: "https://www.getgrist.com/contact",
+  templates: 'https://www.getgrist.com/templates',
   community: 'https://community.getgrist.com',
   functions: 'https://support.getgrist.com/functions',
   formulaSheet: 'https://support.getgrist.com/formula-cheat-sheet',
@@ -647,6 +648,9 @@ export interface GristLoadConfig {
 
   // The Grist deployment type (e.g. core, enterprise).
   deploymentType?: GristDeploymentType;
+
+  // The org containing public templates and tutorials.
+  templateOrg?: string|null;
 }
 
 export const Features = StringUnion(