Move event recurrence controls to a separate page on the workflow

Summary:
Ref T11326. Currently, the "Create Event" form is pretty wordy. One particular culprit is the "recurring" controls, which are (presumably) rarely used and visually complex.

• Reflow the default form to hopefully feel a little better.
• Move recurrence stuff to a separate workflow.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11326

Differential Revision: https://secure.phabricator.com/D16774