rewrite rules: switch to mm when redirecting to secure.gravatar.com
rewrite rules: switch to mm when redirecting to secure.gravatar.com

Gravatar doesn't seem to be able to handle our HTTPS images at the
moment so instead of letting it serve a 504, let's switch to their
mm image in the cases where we would display a Libravatar logo.

libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar