Unauthenticated user sees greyed-out form with signin modal when RSVPing

With this change:
When an unauthenticated user clicks on a Learn/Volunteer RSVP button, they see a greyed out RSVP form with a signin modal.

Before:
When an unauthenticated user clicks on a Learn/Volunteer RSVP button, they see the login modal immediately, and upon signing in, they are redirected to the event page.

Related story: https://www.pivotaltracker.com/story/show/41850205