Adding official Feed stories for Calendar event invite/uninvite actions

Summary: Closes T7988, Adding official Feed stories for Calendar event invite/uninvite actions.

Test Plan: Invite/uninvite users to Calendar events, save, go to Feed, inspect stories.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7988

Differential Revision: https://secure.phabricator.com/D12629