Add user avatars support to the server

[blazern]

1. Figure out best resolution for avatar and max size.
2. Accept user avatar, ensure it's not bigger than max size.
3. Delete old user's avatar from S3 if it exists.
4. Store the avatar on S3, store link to avatar in the user table.
5. Send a URL to user's avatar as a response when user requests user data from server.
6. Delete user avatar from S3 when executing the delete_user cmd.