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.

public/robots.txt

@@ -1,15 +0,0 @@
-# See http://www.robotstxt.org/robotstxt.html for documentation on how to use the robots.txt file
-#
-# To ban all spiders from the entire site uncomment the next two lines:
-# Disallow: /
-
-User-agent: *
-Disallow: /users/
-Disallow: /comments/
-
-Disallow: /*?*locale
-Disallow: /*?*order
-Disallow: /*?*search
-Disallow: /*?*locale-switcher
-Disallow: /*?*filter
-Disallow: user_id