Move closer to upstream ClientIDMutation

[sbor23]

Our Mutation is a homebrew solution to fix some issues with the graphene-django ClientIDMutation.
Now I don't know all the background of this, but #985 shows that we are running into issues with this now, because it seems like the SaveDocumentInput InputType doesn't know about the id which is passed.
See https://travis-ci.com/github/projectcaluma/caluma/jobs/343595361#L2099

The Mutation links three upstream issues that led to the custom implementation: https://github.com/projectcaluma/caluma/blob/master/caluma/caluma_core/mutation.py#L79

Should be considered a show-stopper for #814

Who knows enough about the graphene internals to tackle this?

[sbor23]

It's not as grave is I feared, the specific issue we're facing is tracked here: graphql-python/graphene-django#906

But we should still consider trying to upstream the changes mentioned in said docstrings, the issues on graphene-django are all stale although solutions have been posted.