Finish refactor of edit event/occurrence

[bethlakshmi]

As I was building #1147, I ended up effectively making a new "edit" function to fit with the look and feel and flow of the new create flows.

I want to take the refactor the rest of the way with the following:

• auto detect if this is a volunteer opportunity, and offer a volunteer manager (like we do now) if so.
• manage the case where the event (gbe) is being reused with a new occurrence (scheduler)
• link the new edit event flow into our all events filter
• refactor all make occurrence tests to work with this new code
• delete make_occurrence as dead code