Use the email of a message's sender to create a GravatarUrl profile picture, rendered if there is no AvatarUrl for the message.
Not sure if we want to specify the default image set if there's no gravatar profile picture for the given email, or include a default image in this repo that gravatar can fallback to - either option seems straightforward:
https://en.gravatar.com/site/implement/images/
Use the email of a message's sender to create a GravatarUrl profile picture, rendered if there is no AvatarUrl for the message.
Not sure if we want to specify the default image set if there's no gravatar profile picture for the given email, or include a default image in this repo that gravatar can fallback to - either option seems straightforward: https://en.gravatar.com/site/implement/images/