Add per-tenant sitemap to robots.txt file

While we ping some search engines (currently, only Google) when
generating the sitemap files, we weren't telling search engines
accessing through the `robots.txt` file where to find the sitemap. Now
we're doing so, using the right sitemap file for the right tenant.

spec/system/robots_spec.rb

@@ -0,0 +1,19 @@
+require "rails_helper"
+
+describe "robots.txt" do
+  scenario "uses the default sitemap for the default tenant" do
+    visit "/robots.txt"
+
+    expect(page).to have_content "Sitemap: #{app_host}/sitemap.xml"
+  end
+
+  scenario "uses a different sitemap for other tenants" do
+    create(:tenant, schema: "cyborgs")
+
+    with_subdomain("cyborgs") do
+      visit "/robots.txt"
+
+      expect(page).to have_content "Sitemap: http://cyborgs.lvh.me:#{app_port}/tenants/cyborgs/sitemap.xml"
+    end
+  end
+end