github and gitea: Use html_url as @id and clone_url as codeRepository

They are closer semantics as 'html_url' is the main page of the repository,
so it is the best to identify it; and 'clone_url' is the URL that should
be given to 'git clone', as documented by https://schema.org/codeRepository

Additionally, that property was missing so far; but a future commit will
need to use it to identify fork relationships (node ids are required to
representation relationships between documents as we cannot use blank
nodes for that)