Don't show non-working download button for templates

It's a little followup to recent commit 6c00320. It seems unlikely
that someone would want to disable the download url a public
searchable template, but let's behave sanely if they do.

Also download_url site method instead of preparing the url manually.

Author: simonbaird

rails/app/views/application/_site_tile.html.haml

@@ -20,8 +20,13 @@
   - if clone_buttons
     -# Show the new style clone and download buttons
     .clone-download-buttons
-      = link_to "#{site.url}/download", class: 'btn btn-sm btn-light' do
-        #{bi_icon('download', height: '1.2em', fill: '#777')}Download
+      -#
+        This is an unlikely use case, but the download link won't work if the
+        disable_download_url option is set for a site, so let's not show the
+        download button in that situation
+      - unless site.disable_download_url
+        = link_to site.download_url, class: 'btn btn-sm btn-light' do
+          #{bi_icon('download', height: '1.2em', fill: '#777')}Download
       = link_to new_site_path(clone: site.name), class: 'btn btn-sm btn-light' do
         #{bi_icon('files', height: '1.2em', fill: '#777')}Clone