As #4475 says, MD5 is not available in secure places anymore. This PR
swaps out gravatar-url with an inline function using crypto:sha256 which
is FIPS-140-2 compliant. Since we only used this method for generating
avatar URLs the extra customization wasn't needed and we could hard code
the URL parameters.
fixes: Linear
https://linear.app/unleash/issue/SR-112/gh-support-swap-out-gravatar-url-libcloses: #4475