Use "identicon" gravatars for CritiqueBrainz users
We currently use "mystery-man" gravatar for people who didn't agree to use an email address to retrieve their avatars or simply didn't specify one. It might be better to use "identicon" type gravatars as placeholders instead to make them look more interesting.
To make sure that those "identicon" patterns are consistent for each user we need to pass the same hashes to gravatar's API. See https://en.gravatar.com/site/implement/images/ for more documentation.