Experiment / ExperimentSession integration

[alee]

create server endpoint and wire it up to an interface to allow experimenters to automatically move all signup participants from a given ExperimentSession into a given Experiment

Also look into replacing Experiment.start_date with ExperimentSession.scheduled_date. I think this means that Experiments will have a OneToOne with ExperimentSessions. This makes our data model messy though and allows degeneracy as ExperimentSession still needs the ManyToOne with ExperimentMetadata as not all scheduled subject pool experiment sessions are related to vcweb Experiments.

Consider creating an ExperimentSession automatically when creating an experiment that captures the start date and other metadata.