Fixes #6651,bz1094986 - Disable Sync without repo url

Added code not allow syncing on repositories without a URL.
Could you not do, as I don't think this gives false information to the user:

```
       messages1 = format_bulk_action_messages(
          :success    => "",
          :error      => _("You do not have permissions to sync %s"),
          :models     => @repositories,
          :authorized => @repositories.syncable
        )
  
        messages2 = format_bulk_action_messages(
          :success    => _("Successfully started sync for %s repositories, you are free to leave this page."),
          :error      => _("Repository %s does not have a feed url."),
          :models     => @repositories,
          :authorized => @repositories.has_feed
        )
```