dotcom: redirect sitemap.xml.gz -> GCS

Created by: slimsag

Google should discover our sitemap from the sourcegraph.com/robots.txt entry, but it not existing at sourcegraph.com/sitemap.xml.gz today makes it hard to directly submit it for indexing. Thus, this PR makes sourcegraph.com/sitemap.xml.gz redirect to the GCS bucket which should make it possible for us to submit the sitemap easily via the Google Search Console.

Signed-off-by: Stephen Gutekanst [email protected]