continuous-integration/drone/push Build is passingDetails
This adds the marble avatar from [boring avatars](https://github.com/boringdesigners/boring-avatars) as an option for user avatars. Each user gets a different one (based on their id).
Co-authored-by: kolaente <k@knt.li>
Reviewed-on: vikunja/api#1060
Co-authored-by: konrad <k@knt.li>
Co-committed-by: konrad <k@knt.li>