Add Pinboard Items to Timeline

Summary: This allows adding of pinboard items to a timeline. I'm hoping we can get this in for Maniphest (Pholio, Cover Image) and Macro (because, Macro), but unsure how to scalably do this. Anyways, here's the front end.

Test Plan:
Make some fake timeline items in UIExamples, test mobile, tablet, and desktop breakpoints.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17950