Add X-Robots-Tag header to stop crawlers from indexing media (#8887)

Fixes / related to: https://github.com/matrix-org/synapse/issues/6533

This should do essentially the same thing as a robots.txt file telling robots to not index the media repo. https://developers.google.com/search/reference/robots_meta_tag



Signed-off-by: Aaron Raimist <aaron@raim.ist>