Create separate "Accept" and "Decline" transactions for Calendar

Summary:
Ref T9275. Currently, there's a single "invite" transaction type for managing Calendar invites, and it takes a map of invitees to status.

This isn't great for EditEngine or API access, since it lets you set anyone else to any status and we can't reuse as much code as we can with a simpler API.

Make "Accept" and "Decline" separate actions which affect the actor's invite, so "invite" can be a simpler transaction which just invites or uninvites people.

Test Plan:

• Joined/accepted/declined an event invitation.
• Edited event invitees.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9275

Differential Revision: https://secure.phabricator.com/D16272