Remove libravatar because libravatar.org is shutting down
Implement #94 (closed)
Our libravatar client supports decentralization, but most people probably use the default server which is libravatar.org. There has been interest by volunteers to keep that server running. Anyway this commit removes libravatar in favor of a hardcoded default avatar.
If we decide to continue using libravatar, this MR can be discarded.