Streamline Enrollment Flow Redirects

Currently, each enrollment flow step causes a browser redirect whether or not it does anything (and again for each enroller plugin). This creates a lot of browser spinning ("Please wait, executing step foo...").

Consider calculating the next step within CoPetitionsController, possibly with the help of Plugin calls to determine whether or not the Plugin wants to run. Then only redirect when a page needs to render.

Maybe use the Event system for this? Events fire until something says "I need to render".