Convert "Created By" and "Invited" inputs in Calendar search to use PhabricatorPeopleUserFunctionDatasource

Summary: Closes T8046, Convert "Created By" and "Invited" inputs in Calendar search to use PhabricatorPeopleUserFunctionDatasource

Test Plan: Calendar → Advanced Search, search for "members:Calendar", created by and invited. Should autofill and search correctly.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8046

Differential Revision: https://secure.phabricator.com/D13311