Add new participants on intervention creation

Allow participant assignment that is available on edit to also be available on intervention creation.

1. Excercise this with a feature test, use the same technique I did to provide an alternative to the rjs. (I'll walk you through this.) (I did an example of this in the sims demo walkthrough feature, search for creating and editing score #195)

For the alternative to RJS note

http://github.com/vegantech/sims...

in particular app/views/interventions/_intervention_probe_assignment_detail.html.erb

The href and the param (and subsequent check in the view) This is a means of accessing the same functionality without rjs so we can test it. (And we can run it in the browser without javascript.)

1. Make the features pass using TDD to drive changes to the controller and model (and the corresponding specs/controllers specs/models)

2. Confirm(with a test) that the email that is sent out goes to the added participants.