🐛 Removed redirects from search engine indexing (#15617)

refs TryGhost/Product#2072

Google is indexing our redirects and storign the redirected content
against the redirect URL in search results. This seems to be caused by
us using a 302 redirect rather than 301. We don't want to switch to a
301 however, so that we can support the ability to update redirects in
the future.

ghost/core/core/frontend/public/robots.txt

@@ -3,3 +3,4 @@ Sitemap: {{blog-url}}/sitemap.xml
 Disallow: /ghost/
 Disallow: /p/
 Disallow: /email/
+Disallow: /r/

ghost/core/test/e2e-frontend/default_routes.test.js

@@ -326,7 +326,8 @@ describe('Default Frontend routing', function () {
  'User-agent: *\n' +
  'Sitemap: http://127.0.0.1:2369/sitemap.xml\nDisallow: /ghost/\n' +
  'Disallow: /p/\n' +
- 'Disallow: /email/\n'
+ 'Disallow: /email/\n' +
+ 'Disallow: /r/\n'
  );
  });