R1:667897639fe2

First pass at event modal create options.

Summary: Ref T8568, First pass at event modal create options.

Test Plan: {nav Calendar > Create} should offer three options: Create Event, Create Public Event, Create Recurring Event.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T8568

Differential Revision: https://secure.phabricator.com/D13309