Experiment workspaces: allow "-" in experiment_name?

Every time I create an experiment workspace, I forget that experiment_name can't contain hyphens, which feels like a very unnatural restriction to me. I know that we construct the new workspace name by joining the base workspace name and the experiment name with a hyphen, which is probably where this restriction came from; but I don't think we rely on being able to decompose that again, so it shouldn't really matter. Could we perhaps add "-" to the list of valid characters here?

/cc @enrico