Skeleton of calendar icon infrastructure

Summary: Ref T7936, Skeleton of calendar icon infrastructure

Test Plan: Navigate to install/calendar/icon/, observe temporary calendar icons

Reviewers: epriestley, #blessed_reviewers, chad

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7936

Differential Revision: https://secure.phabricator.com/D12928