Create new ExperimentSubject model with user_id, group_id, or project_id

Create a new ExperimentSubject model and the associated experiment_subjects table. Include all of the columns from the existing experiment_users table, including the user_id column, and also add group_id and project_id columns. Add a constraint which stipulates that at least one of user_id, group_id, or project_id must have a value.

In more detail:

• create a new experiment_subjects table, leave the existing experiment_users table as-is
• give the new experiment_subjects table the same columns as the existing experiment_users table
• give the new experiment_subjects table additional columns for group_id and project_id
• add a constraint to the new experiment_subjects table which stipulates that at least one of user_id, group_id, or project_id must have a value
• create a new ExperimentSubject model
• add an Experiment#has_many :experiment_subjects relationship