Allow volunteer admins to save a draft schedule

Admins may want to try out different configurations of volunteers to see how well they can cover their needs by moving people around. The current save-on-drop architecture discourages that somewhat because every experiment is instantly saved, with no obvious way of reverting.

Since the project entity is our container, one way to provide easy rollback is to copy all of the scheduled activities into a new subproject (i.e., entity_table is civicrm_volunteer_project) for drafting. Once the admin decides to use the draft, the activities from the draft are copied into the parent project.